Sleep-Related Nighttime Crying (Yonaki) in Japan: A Community-Based Study
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《小儿科》
Department of Child Neurology, National Institute of Neuroscience, National Center of Neurology and Psychiatry, Kodaira, Tokyo, Japan
Department of Psychology, University of Maine, Orono, Maine
Department of Pediatrics, Faculty of Medicine, Tokyo Medical and Dental University, Tokyo, Japan
ABSTRACT
Objective. To examine factors associated with the phenomenon of yonaki, or sleep-related nighttime crying (SRNC), in Japanese children
Methods. A cross-sectional design incorporating parental self-report was used to investigate relationships between developmental, psychologic, and constitutional/physiological factors in the incidence of SRNC. Participants were the parents of 170 infants, 174 toddlers, and 137 children at a well-infant clinic in Tokyo, Japan.
Results. The lifetime incidence rates of SRNC were 18.8% (infants), 64.9% (toddlers), and 59.9% (children). At all ages, children were most likely to cosleep with their parents; however, infants with reported SRNC were found to cosleep more frequently, whereas infants without SRNC were more likely to sleep in separate, child-dedicated beds. Toddlers with frequent SRNC were more likely to have irregular bedtimes and to have nonparental day care than were those without SRNC. Preschoolers who typically slept 9.5 to 10.5 hours per night were less likely to report SRNC than were children with longer or shorter nighttime sleep durations. In all groups, children with frequent SRNC were more likely to suffer from chronic eczema, and toddlers and preschoolers with SRNC exhibited bruxism more frequently.
Conclusions. The traditional Japanese arrangement of cosleeping represents an environment in which parents are readily accessible to children during waking episodes. Physical proximity to the parents in infancy, but not at other ages, is associated with SRNC. The higher incidence of bruxism, chronic eczema, and day care use among children with frequent SRNC supports the hypothesis that nighttime anxiety may promote SRNC.
Key Words: sleep-related childhood nighttime crying night waking cosleeping nighttime anxiety day care bruxism infant children
Abbreviations: SRNC, sleep-related nighttime crying GA, gestational age
In Japanese, sleep-related nighttime crying (SRNC) is called yonaki, yo- meaning night and -naki meaning crying. SRNC occurs most often after midnight, when infants have been asleep for some time, in contrast to colic, which peaks in the late afternoon and early evening. In a Japanese survey of parents, SRNC was reported to emerge by 4 to 24 months of age in most children.1 Although SRNC is a frequent complaint of parents, it is considered by most pediatricians to be a benign phenomenon that disappears naturally over time. Few studies of SRNC have been performed, however, and it does create sleep deficit for both parents and children in its persistent form.1
Young children in Japan, as in the West, frequently cry, have tantrums, or misbehave at bedtime and often experience distressed waking at night from which they do not resettle without parental intervention.2–6 The prevalence of such settling and sleep disturbances ranges from 20% to as high as 42% in Western countries.3–7 It is well known that various factors impact the development of persistent night waking in infants, toddlers, and preschoolers.8 These factors include features of the child's sleep/wake rhythm,9,10 maternal psychologic state,11,12 infant temperament,10,12–14 sleep environment, use of transitional objects,10 and sleeping arrangement.14–16 This study investigated these and other factors that have been associated with arousals and awakenings, as well as their association with parental descriptions of SRNC.
MATERIALS AND METHODS
Participants
Participants were recruited through 1 of the authors (M.F.), who is also an attending pediatrician in the developmental clinic of the Higashimurayama city public health center in Tokyo, Japan. Parents of 429 healthy infants (3–6 months old), 452 toddlers (18–21 months old), and 440 preschool-aged children (36–41 months old) were asked to participate in the study and given questionnaires by the principal investigator during well-child checkups. The purpose of the survey was explained to staff and parents. Informed consent was received from all parents. The study conformed to all international standards of research with human subjects; permission to conduct the study was sought from the Director of the National Center Hospital in accordance with standard Japanese ethical research practices.1,17 The study was conducted between October 2001 and March 2002. The questionnaires were anonymous (ie, not coded for identity of child or family). Parents filled out the questionnaires at home and returned them by mail. All data were collected in the fall and winter months.
Measures
Questionnaire
The format of the questionnaire included Likert-style questions (7 items) and forced-choice (15 items), multiple-choice (6 items), and fill-in (7 items) questions. Parents were queried on many aspects of sleep, including arousals and awakenings during the night, bedtime routine (eg, whether they played or talked with their children before bed, bathroom activities, changing diapers, carrying or other physical contact, feeding or drinking, music, or other entertainment), and sleep habits. The parents were asked whether their children had experienced SRNC at any time either currently or in the past. For the purposes of this study, SRNC was defined as unexplained awakening from sleep characterized by crying that usually occurred every day. This definition was reiterated to the parents when the forms were given out. Parents were asked to evaluate whether SRNC had ever occurred in the child's lifetime; responses represent retrospective estimates. Parents of preschool-aged children with histories of SRNC were asked whether SRNC had disappeared by the time of the survey. The questionnaire content was otherwise the same for all 3 age groups. Itemized details of the questionnaire are presented in the Appendix.
APPENDIX. Questionnaire Items
Demographics
Demographic and housing information was collected and used as proxy variables to estimate socioeconomic status based on the cost of housing. In Japan, housing is quite expensive, especially private houses and condominiums. Generally, houses and condominiums are occupied by families who can afford the down payment and mortgage and are, therefore, of higher socioeconomic status. Families that rent apartments do so typically because they cannot afford the cost of houses or condominiums. In our questionnaires, we did not ask about annual income, because direct questions regarding income are not allowed in Japan.
Parents were asked about their marital status and family structure. A nuclear family was defined as parents and children living without other family members. Extended family was defined as parents and children living with other family members.
Constitutional and Other Sleep Factors
Children's medical histories were explored, including method of delivery, any perinatal abnormalities, and past illnesses. Specific questions addressed chronic eczema (atopic dermatitis), asthmatic bronchitis, speech delay, and motor delay, because these conditions are known to be associated with sleep disorders. Children's exposure to second-hand smoke was assessed also.
Questions were asked regarding bruxism, snoring, involuntary movements, or any other unusual behaviors during sleep. Parasomnias were not specifically defined and therefore not analyzed.
Psychologic Factors
Parents were asked about the presence and estimated amount of frequent daytime crying. In the infant group, colic (as distinct from SRNC) was defined as a positive response to the question: "Does your infant cry frequently in the daytime (including the evening)" A second question evaluated the prevalence of crying: "Did your infant cry for >3 hours during the daytime"18,19 If the parents responded to either question positively, the child was coded as suffering from colic.
Children's preferences for specific objects (preferred objects during the day and the use of sleep aids or transitional objects at night) were explored. Parents were requested to assess their children's reactions to unfamiliarity in the social or physical environment and to describe their children's behavioral styles.
Nonparental and other aspects of child care were explored in the context of which family member was most involved in day-to-day child care, how strict the parents were, and how enjoyable they found child rearing. The frequency and pattern of out-of-home day care use also were examined for each age group.
Other Factors Affecting Sleep
The presence and level of ambient environmental noise (eg, street traffic, airplane noise) and light, especially during the night, were estimated, as were the frequency, time, and duration of naps and of daily outdoor light exposure. Usual bedtimes, typical duration of nighttime sleep, and parents' estimates of sleep regularity were elicited. Several questions addressed the issue of children's sleeping location or sleeping space, touching on whether the child slept in the same room as the parents, whether the child "coslept" in the same bed as the parents, and, if cosleeping took place, whether this was the norm or occurred intermittently and whether the current pattern had always applied.
Statistical Analysis
Analysis of the difference in frequency across groups was performed with the 2 test. Fisher's exact test was used only in 2-category or 2-group comparisons in cases where expected frequency was <5. Mann-Whitney, Kruskal-Wallis, and Friedman's nonparametric tests were used to examine differences between groups on some measures. Multiple logistic regression (SAS statistical software, SAS Institute, Inc, Cary, NC) was used to assess the relationships of sleeping arrangements and other demographic and health factors to the incidence of SRNC.
RESULTS
Demographic characteristics of the respondents are shown in Table 1. The rate of questionnaire return was 39.6% (170 of 429) by parents of infants, 38.5% (174 of 452) by parents of toddlers, and 31.1% (137 of 440) by parents of preschoolers. Parents with multiple children filled out separate questionnaires for each child. A total of 481 questionnaires were returned (36%) of the 1321 that were distributed.
There were very few single-parent families, and only-child families were the most common in all groups (P < .001), as were nuclear families. The average ages of the parents, as would be expected, increased with increasing age of the children's groups (P < .001). There were no between-group differences in housing style.
There were 5 (2.9%) premature infants (35–36 weeks' gestational age [GA]), 9 (5.2%; 34–36 weeks' GA) in the toddler group, and 5 (3.8%; 27–36 weeks' GA) in the preschool group. Across groups, the range of GA for other children was 37 to 42 weeks. The rates of cesarean section were 11% for the infant group, 16% for the toddler group, and 9% for the preschooler group. The infant and preschooler groups each had 1 set of twins. The toddler group had 2 set of twins. Evidence of developmental delay, defined as "speech and/or motor slowness," was identified in 2.9% of infants, 6.3% of toddlers, and 5.1% of preschoolers. Parents did not report any chronic major illnesses except asthma and eczema in any of the groups.
SRNC and Frequent Daytime Crying
The definition of excessive crying in the infant group met the criteria for colic as described in the literature. The rate of "colic-age" frequent crying in the infant group was 8.2% (14 of 170), which is close to general rates of colic at this age. Only 1 infant was reported to have both colic and SRNC. Among toddlers with SRNC, the rate of those whose parents considered them to be crying frequently during the day at the time of the survey was 34.9% (38 of 109), significantly higher than that for children described as crying frequently but without SRNC (8.6% [11 of 59]; P < .05). Among preschoolers, children with SRNC were not different in rates of current frequent crying from those without SRNC.
SRNC and Preferred Objects
Preferred objects were those with which children chose to play during the day. The rates of use of stuffed animals or puppets by both toddlers and preschoolers were significantly higher than that of infants (P < .005). The rates of use of all preferred objects were not significantly different between those with and without SRNC.
Sleep aids were objects that children used during the wake/sleep transition. For infants and toddlers, the use rates of sleep aids were not different between those with and without SRNC. For preschoolers, more of those with than without SRNC were reported as always or sometimes using objects as sleep aids (97.4% [38 of 39] vs 80.8% [21 of 26]; P < .05). Infants and toddlers did not show preferences for object type in relation to SRNC, but more preschoolers with than without SRNC preferred objects such as toys (28.9% [11 of 38] vs 4.8% [1 of 21]; P < .05).
SRNC and Sleeping Arrangements During Infancy
It should be noted that in the typical Japanese sleeping arrangement, parents and children sleep next to each other on mattresses or futons on the floor or on adult beds. The Japanese crib is defined as a dedicated "child bed," similar to a Western-style crib, that is enclosed with a headboard separating parents and children; the parents typically sleep immediately next to the child bed on another bed or futon. Alternatively, the child shares the adult bed or futon with the parents. The child bed is primarily used in infancy and is thought by Japanese parents to be particularly protective in the vulnerable neonatal period, because parents are concerned about the possibility of rolling over or otherwise hurting vulnerable neonates if they share the same bed.
The majority of infants (47.3%) slept on mattresses or futons; 80% of these coslept with their parents. Dedicated child beds were used exclusively by 37.3% of infants; of these, 52.4% usually coslept for part of the night. The rate of cosleeping among infants using child beds was significantly lower than in the adult-bed (P < .005) and futon (P < .001) groups. In the infant group only, children without SRNC were significantly more likely to sleep in separate, dedicated child beds than were those with SRNC (P < .02). The amount and pattern of cosleeping was examined in 3 categories: all night, during sleep onset and/or night waking, and during naptime. There were no between-category differences in SRNC.
SRNC and Sleep Patterns
Napping was not calculated for young infants, because pilot observations showed that parents could not identify stable patterns for children 2 to 3 months old. For older groups, the reported rate of daytime napping was 100% (n = 174) for toddlers and 72.3% (99 of 137) for preschoolers. Among toddlers, 2 children napped twice each day.
With the exception of children whose naptime was unstable or who napped twice per day, the average toddler nap began at 1:00 PM (n = 125), and the average preschooler nap began at 1:48 PM (n = 71). These data suggest that the naptime sleep phase was significantly delayed in preschoolers compared with toddlers (P < .001). In both groups, the incidence of irregular or late naptime was not related to SRNC status. The average nap duration for toddlers was 2.0 ± 0.6 hours (n = 167). Except for those with unstable nap durations and those who did not take naps, average nap duration was 1.8 ± 0.5 hours for preschoolers (n = 94). SRNC status was not related to nap duration; however, there was a significant decline in the duration of naps from toddler to preschool years (P < .005).
Parents were asked about their children's regularity of sleep onset. The average bedtime for toddlers and preschoolers was 9:36 PM (n = 162) and 9:30 PM (n = 125), respectively, ±1 hour. There were no significant differences in night sleep-onset time between these 2 groups. Among toddlers, significantly more (9.8% [11 of 112]) of those with but none of those without SRNC showed bedtime irregularity (P < .01). Among preschoolers, SRNC was not associated with bedtime irregularity. Late bedtime was not related to SRNC status in either group.
A "stable sleeper" was defined by the parental perception that a child usually slept the same amount each night. The average duration of nighttime sleep was not significantly different between toddlers (98.2% [168 of 171]; 9.8 ± 1.1 hour) and preschoolers (97.8% [133 of 136]; 10.1 ± 1.0 hours) who were stable sleepers. Preschoolers without SRNC were more likely to be stable sleepers that those with SRNC (P < .05).
A definition of "average" duration of sleep (9.5–10.5 hours) was also created based on the average across the toddler (9.8 hours) and preschool (10.1 hour) groups of stable sleepers in the sample. Preschoolers with SRNC (44.3% [35 of 79]) were less likely to have average sleep amounts, defined as sleep duration that was within the 9.5- to 10.5-hour range, than were preschoolers without SRNC (P < .5). There were no significant SRNC-related differences in stable sleepers and children with average sleep duration among toddlers.
SRNC and Other Factors
Family Structure and Child Care
Parent ratings of children's attractiveness as social partners, parent enjoyment of child rearing, and parent strictness in child rearing did not differ across age groups or with SRNC status. Among toddlers only, SRNC was significantly more common in nuclear families than in extended families (94.7% [107 of 113] vs 85.2% [52 of 61], respectively; P < .05). Also among toddlers only, 42.6% (48 of 113) of those without SRNC were rated as "cautious/wary" by parents. This trait, also translated as "carefulness," is viewed positively by parents and is thought to be associated with maturity. Toddlers without SRNC were significantly more likely to be so rated than were those with SRNC (P < .005). There were no SRNC-related differences in reactivity to unfamiliar situations.
Chronic Conditions
Among infants, toddlers, and preschoolers with SRNC, 35.7% (5 of 14), 81.3% (13 of 16), and 62.5% (10 of 16), respectively, suffered from chronic eczema. Across all groups, children with SRNC (12.4% [28 of 226]) were significantly more likely to have chronic eczema than were those without SRNC (7.1% [18 of 254]; P < .05).
The rate of SRNC with snoring was 44.4% (4 of 9) for infants, 70% (7 of 10) for toddlers, and 63.6% (7 of 11) for preschoolers. Across all groups, there were no differences in snoring between children with (7.9% [18 of 227]) and without (4.7% [12 of 254]) SRNC.
Of toddlers and preschoolers, 69.2% (9 of 13) and 80% (12 of 15), respectively, suffered from asthma. No cases of SRNC and asthma (or SRNC and bruxism) were reported among infants. There was no significant difference in the rate of SRNC with (10.8% [21 of 194]) and without (6.0% [7 of 116]) asthma. All toddlers (7 of 7) identified with bruxism had histories of SRNC, as did 90% (9 of 10) of preschoolers with SRNC. Overall, children with SRNC (8.2% [16 of 195]) had significantly more bruxism than did those without SRNC (0.9% [1 of 116]; P < .005).
Cosleeping and Child Bed Use in Infancy
Complex sleeping arrangements were seen only in infancy; almost all the toddlers and preschoolers coslept in or next to the adult bed. However, in infancy, cosleeping and child bed use were not exclusive categories (ie, partial cosleeping in the child bed was observed in some cases). Hence, 4 sleeping-arrangement categories were examined: (1) child bed without cosleeping; (2) child bed with cosleeping; (3) adult bed/futon or mixed pattern with cosleeping; and (4) adult bed/futon or mixed pattern without cosleeping. In the final analysis, stepwise, multiple logistic regression was used to examine sleeping arrangements, family structure, nonparental care, and chronic eczema as predictors of the development of SRNC during infancy. Only category 3 sleeping arrangements significantly predicted SRNC risk (P < .006). Hence, specifically during infancy, the combination of cosleeping and adult bed/futon or mixed-pattern sleeping was strongly associated with the development of SRNC.
DISCUSSION
Children who cosleep with their parents are known to have a higher prevalence of sleep problems, especially night waking.14–16,20 The results of our study suggest that the incidence of SRNC is related to sleeping arrangement, especially to patterns of cosleeping in infancy, as well as to individual characteristics of children and families. Lozoff and co-workers16 found that cosleeping per se was not associated with increased sleep problems in early childhood; night-waking levels were similar between cosleeping Japanese children and US children sleeping separately. In our study, Japanese parents and children coslept routinely at all the ages examined, but patterns among infants had subtle, important differences from those observed in the toddler and preschool groups.
In Western countries, sleep disorders have been found to be both common and persistent in early childhood. Of children with sleep problems at 8 months of age, 32% to 41% were observed to have continuing problems at 3 and 4 years of age.12,21 The results of our study suggest that the toddler age range is the most likely phase for development of SRNC. Parent reports of positive SRNC histories increased dramatically from infancy to toddlerhood but did not change between the toddler and preschool years. In fact, we found that SRNC had disappeared in 80% of children by the preschool years. We estimate that 60% of children had experienced SRNC by preschool age.
In agreement with previous work on the association of night waking and cosleeping, the phenomenon of SRNC seems to be related to traditional Japanese sleeping arrangements.20 In our study, the type of sleeping arrangement used in infancy was associated with SRNC. Specifically, infants who slept in dedicated child beds were much less likely to have SRNC than were those who had other arrangements. Infants whose parents endorsed cosleeping and who slept in or next to the parents' beds accounted for a threefold increase in risk for SRNC.
Regardless of the sleeping arrangement used, Japanese parents often cosleep on children's beds for the wake/sleep transition and sometimes after awakenings during the night,22 usually moving to their own beds after the children fall asleep. Anders et al23 reported that infants who were put into cribs already asleep at the beginning of the night were significantly more likely to develop the "signaling" pattern that alerts parents to night wakings by crying after spontaneous awakenings. Additionally, infants who developed these learned sleep-onset associations required parental intervention more often than did those who were put into their cribs awake at bedtime and allowed to fall asleep on their own. In an earlier study, we found that falling asleep in the parents' bed after night waking in infancy, particularly among breastfed infants, was associated with sleep disturbance and requests for cosleeping in early childhood.24 We suggest that SRNC may be governed by the same mechanism (ie, cosleeping in infancy encourages SRNC), whereas separate sleeping does not. Furthermore, parents are more likely to be aware of nighttime awakenings when infants sleep in close proximity to them.
The child bed was primarily used only during infancy. At toddlerhood, there was an apparent shift toward cosleeping, a pattern that, according to our data, was retained through the preschool years. Depending on family and child characteristics, it is typical to promote independent sleeping in another bed in the same or a different room at the beginning of elementary school. This may be related to SRNC incidence, because >80% of SRNC had disappeared by preschool age in our study.
In contrast to one frequently posed hypothesis,25,26 a well-controlled study found no association between colic and temperamental compromise.25,27 The developmental perspective is that neither colic nor SRNC is related to psychologic dysfunction or other outcome risk.1,25,28 In our study, the age range of the infant group was old for colic.29 Nonetheless, Clifford et al30 reported a prevalence of colic at 3 months of 6.4%, similar to the rate for SRNC (8.2%) that we observed.
Children with persistent sleep difficulties have been found to be more likely to have behavior problems, especially with tantrums and behavior management.13 In our study, toddlers who did not have SRNC were more likely to be reported to have the culturally desirable carefulness or cautious/wary trait than were those with SRNC, and toddlers who cried frequently during the day were more likely to have SRNC. Immature regulation of emotionality and/or anxiety, as represented by daytime temperamental characteristics, may be linked to immaturity of sleep-consolidation mechanisms.
SRNC symptoms seemed to be most frequent in toddlerhood. Interestingly, nonparental care in the toddler group, but not in other age groups, was associated with SRNC. It is important to note that in Japan, the reputations of day care centers, as in other industrialized countries, are variable, and day care quality was not assessed in our study. To date, there have been few reports concerning nonparental care and sleep problems.31 Nonetheless, day care was most commonly begun at the toddler age, and the associated change in routine from individual care may have increased nighttime anxiety at least initially.12 It is known that separation anxiety is related to night waking.32 In our study, toddlers, but not the other groups, showed SRNC significantly more commonly in nuclear families than in extended families, which suggests that toddlers may be more reactive to environmental changes or conditions.
We found that bruxism and atopic dermatitis were more common in toddlers and preschoolers with SRNC. With mild atopic dermatitis, awakening seems to be induced easily by itching.33 Atopic disorders in children have also been found to be associated with separation anxiety.34 Children older than our age groups with atopic dermatitis in clinical remission have been reported to have more frequent arousals and awakenings that are not related to scratching per se.33 Sleep bruxism is a relatively common childhood parasomnia and has been associated with high anxiety scores.35 Our findings support the hypothesis that frequent behavioral arousal and nighttime anxiety may contribute to SRNC. In addition, parents of children with chronic conditions such as eczema may have higher levels of concern and thus be more attentive to nighttime symptoms.
Western cultural practices of nonparental care are becoming increasingly common in Japan, as is the pressure on parents to meet daytime work demands. Coping with children who habitually wake, cry, and disturb their own and their parents' sleep is an important issue in child development and family health. This study was limited by being based on retrospective parent reports that were not objectively corroborated and by the severity and duration of SRNC not being ascertained. Nevertheless, the results suggest that infancy patterns of parent-child nighttime interactions and sleeping arrangements may provide guidance in the management and prevention of persistent night waking in early childhood.
ACKNOWLEDGMENTS
Special thanks to the staff of the Higashimurayama city public health centers for their generous cooperation; Alan Rosenwasser for comments on earlier versions of the manuscript; Patricia Hofmaster for consultation on biostatistical analyses; and Kathleen McAuliffe for help in preparation of the manuscript.
FOOTNOTES
Accepted Aug 5, 2004.
No conflict of interest declared.
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Department of Psychology, University of Maine, Orono, Maine
Department of Pediatrics, Faculty of Medicine, Tokyo Medical and Dental University, Tokyo, Japan
ABSTRACT
Objective. To examine factors associated with the phenomenon of yonaki, or sleep-related nighttime crying (SRNC), in Japanese children
Methods. A cross-sectional design incorporating parental self-report was used to investigate relationships between developmental, psychologic, and constitutional/physiological factors in the incidence of SRNC. Participants were the parents of 170 infants, 174 toddlers, and 137 children at a well-infant clinic in Tokyo, Japan.
Results. The lifetime incidence rates of SRNC were 18.8% (infants), 64.9% (toddlers), and 59.9% (children). At all ages, children were most likely to cosleep with their parents; however, infants with reported SRNC were found to cosleep more frequently, whereas infants without SRNC were more likely to sleep in separate, child-dedicated beds. Toddlers with frequent SRNC were more likely to have irregular bedtimes and to have nonparental day care than were those without SRNC. Preschoolers who typically slept 9.5 to 10.5 hours per night were less likely to report SRNC than were children with longer or shorter nighttime sleep durations. In all groups, children with frequent SRNC were more likely to suffer from chronic eczema, and toddlers and preschoolers with SRNC exhibited bruxism more frequently.
Conclusions. The traditional Japanese arrangement of cosleeping represents an environment in which parents are readily accessible to children during waking episodes. Physical proximity to the parents in infancy, but not at other ages, is associated with SRNC. The higher incidence of bruxism, chronic eczema, and day care use among children with frequent SRNC supports the hypothesis that nighttime anxiety may promote SRNC.
Key Words: sleep-related childhood nighttime crying night waking cosleeping nighttime anxiety day care bruxism infant children
Abbreviations: SRNC, sleep-related nighttime crying GA, gestational age
In Japanese, sleep-related nighttime crying (SRNC) is called yonaki, yo- meaning night and -naki meaning crying. SRNC occurs most often after midnight, when infants have been asleep for some time, in contrast to colic, which peaks in the late afternoon and early evening. In a Japanese survey of parents, SRNC was reported to emerge by 4 to 24 months of age in most children.1 Although SRNC is a frequent complaint of parents, it is considered by most pediatricians to be a benign phenomenon that disappears naturally over time. Few studies of SRNC have been performed, however, and it does create sleep deficit for both parents and children in its persistent form.1
Young children in Japan, as in the West, frequently cry, have tantrums, or misbehave at bedtime and often experience distressed waking at night from which they do not resettle without parental intervention.2–6 The prevalence of such settling and sleep disturbances ranges from 20% to as high as 42% in Western countries.3–7 It is well known that various factors impact the development of persistent night waking in infants, toddlers, and preschoolers.8 These factors include features of the child's sleep/wake rhythm,9,10 maternal psychologic state,11,12 infant temperament,10,12–14 sleep environment, use of transitional objects,10 and sleeping arrangement.14–16 This study investigated these and other factors that have been associated with arousals and awakenings, as well as their association with parental descriptions of SRNC.
MATERIALS AND METHODS
Participants
Participants were recruited through 1 of the authors (M.F.), who is also an attending pediatrician in the developmental clinic of the Higashimurayama city public health center in Tokyo, Japan. Parents of 429 healthy infants (3–6 months old), 452 toddlers (18–21 months old), and 440 preschool-aged children (36–41 months old) were asked to participate in the study and given questionnaires by the principal investigator during well-child checkups. The purpose of the survey was explained to staff and parents. Informed consent was received from all parents. The study conformed to all international standards of research with human subjects; permission to conduct the study was sought from the Director of the National Center Hospital in accordance with standard Japanese ethical research practices.1,17 The study was conducted between October 2001 and March 2002. The questionnaires were anonymous (ie, not coded for identity of child or family). Parents filled out the questionnaires at home and returned them by mail. All data were collected in the fall and winter months.
Measures
Questionnaire
The format of the questionnaire included Likert-style questions (7 items) and forced-choice (15 items), multiple-choice (6 items), and fill-in (7 items) questions. Parents were queried on many aspects of sleep, including arousals and awakenings during the night, bedtime routine (eg, whether they played or talked with their children before bed, bathroom activities, changing diapers, carrying or other physical contact, feeding or drinking, music, or other entertainment), and sleep habits. The parents were asked whether their children had experienced SRNC at any time either currently or in the past. For the purposes of this study, SRNC was defined as unexplained awakening from sleep characterized by crying that usually occurred every day. This definition was reiterated to the parents when the forms were given out. Parents were asked to evaluate whether SRNC had ever occurred in the child's lifetime; responses represent retrospective estimates. Parents of preschool-aged children with histories of SRNC were asked whether SRNC had disappeared by the time of the survey. The questionnaire content was otherwise the same for all 3 age groups. Itemized details of the questionnaire are presented in the Appendix.
APPENDIX. Questionnaire Items
Demographics
Demographic and housing information was collected and used as proxy variables to estimate socioeconomic status based on the cost of housing. In Japan, housing is quite expensive, especially private houses and condominiums. Generally, houses and condominiums are occupied by families who can afford the down payment and mortgage and are, therefore, of higher socioeconomic status. Families that rent apartments do so typically because they cannot afford the cost of houses or condominiums. In our questionnaires, we did not ask about annual income, because direct questions regarding income are not allowed in Japan.
Parents were asked about their marital status and family structure. A nuclear family was defined as parents and children living without other family members. Extended family was defined as parents and children living with other family members.
Constitutional and Other Sleep Factors
Children's medical histories were explored, including method of delivery, any perinatal abnormalities, and past illnesses. Specific questions addressed chronic eczema (atopic dermatitis), asthmatic bronchitis, speech delay, and motor delay, because these conditions are known to be associated with sleep disorders. Children's exposure to second-hand smoke was assessed also.
Questions were asked regarding bruxism, snoring, involuntary movements, or any other unusual behaviors during sleep. Parasomnias were not specifically defined and therefore not analyzed.
Psychologic Factors
Parents were asked about the presence and estimated amount of frequent daytime crying. In the infant group, colic (as distinct from SRNC) was defined as a positive response to the question: "Does your infant cry frequently in the daytime (including the evening)" A second question evaluated the prevalence of crying: "Did your infant cry for >3 hours during the daytime"18,19 If the parents responded to either question positively, the child was coded as suffering from colic.
Children's preferences for specific objects (preferred objects during the day and the use of sleep aids or transitional objects at night) were explored. Parents were requested to assess their children's reactions to unfamiliarity in the social or physical environment and to describe their children's behavioral styles.
Nonparental and other aspects of child care were explored in the context of which family member was most involved in day-to-day child care, how strict the parents were, and how enjoyable they found child rearing. The frequency and pattern of out-of-home day care use also were examined for each age group.
Other Factors Affecting Sleep
The presence and level of ambient environmental noise (eg, street traffic, airplane noise) and light, especially during the night, were estimated, as were the frequency, time, and duration of naps and of daily outdoor light exposure. Usual bedtimes, typical duration of nighttime sleep, and parents' estimates of sleep regularity were elicited. Several questions addressed the issue of children's sleeping location or sleeping space, touching on whether the child slept in the same room as the parents, whether the child "coslept" in the same bed as the parents, and, if cosleeping took place, whether this was the norm or occurred intermittently and whether the current pattern had always applied.
Statistical Analysis
Analysis of the difference in frequency across groups was performed with the 2 test. Fisher's exact test was used only in 2-category or 2-group comparisons in cases where expected frequency was <5. Mann-Whitney, Kruskal-Wallis, and Friedman's nonparametric tests were used to examine differences between groups on some measures. Multiple logistic regression (SAS statistical software, SAS Institute, Inc, Cary, NC) was used to assess the relationships of sleeping arrangements and other demographic and health factors to the incidence of SRNC.
RESULTS
Demographic characteristics of the respondents are shown in Table 1. The rate of questionnaire return was 39.6% (170 of 429) by parents of infants, 38.5% (174 of 452) by parents of toddlers, and 31.1% (137 of 440) by parents of preschoolers. Parents with multiple children filled out separate questionnaires for each child. A total of 481 questionnaires were returned (36%) of the 1321 that were distributed.
There were very few single-parent families, and only-child families were the most common in all groups (P < .001), as were nuclear families. The average ages of the parents, as would be expected, increased with increasing age of the children's groups (P < .001). There were no between-group differences in housing style.
There were 5 (2.9%) premature infants (35–36 weeks' gestational age [GA]), 9 (5.2%; 34–36 weeks' GA) in the toddler group, and 5 (3.8%; 27–36 weeks' GA) in the preschool group. Across groups, the range of GA for other children was 37 to 42 weeks. The rates of cesarean section were 11% for the infant group, 16% for the toddler group, and 9% for the preschooler group. The infant and preschooler groups each had 1 set of twins. The toddler group had 2 set of twins. Evidence of developmental delay, defined as "speech and/or motor slowness," was identified in 2.9% of infants, 6.3% of toddlers, and 5.1% of preschoolers. Parents did not report any chronic major illnesses except asthma and eczema in any of the groups.
SRNC and Frequent Daytime Crying
The definition of excessive crying in the infant group met the criteria for colic as described in the literature. The rate of "colic-age" frequent crying in the infant group was 8.2% (14 of 170), which is close to general rates of colic at this age. Only 1 infant was reported to have both colic and SRNC. Among toddlers with SRNC, the rate of those whose parents considered them to be crying frequently during the day at the time of the survey was 34.9% (38 of 109), significantly higher than that for children described as crying frequently but without SRNC (8.6% [11 of 59]; P < .05). Among preschoolers, children with SRNC were not different in rates of current frequent crying from those without SRNC.
SRNC and Preferred Objects
Preferred objects were those with which children chose to play during the day. The rates of use of stuffed animals or puppets by both toddlers and preschoolers were significantly higher than that of infants (P < .005). The rates of use of all preferred objects were not significantly different between those with and without SRNC.
Sleep aids were objects that children used during the wake/sleep transition. For infants and toddlers, the use rates of sleep aids were not different between those with and without SRNC. For preschoolers, more of those with than without SRNC were reported as always or sometimes using objects as sleep aids (97.4% [38 of 39] vs 80.8% [21 of 26]; P < .05). Infants and toddlers did not show preferences for object type in relation to SRNC, but more preschoolers with than without SRNC preferred objects such as toys (28.9% [11 of 38] vs 4.8% [1 of 21]; P < .05).
SRNC and Sleeping Arrangements During Infancy
It should be noted that in the typical Japanese sleeping arrangement, parents and children sleep next to each other on mattresses or futons on the floor or on adult beds. The Japanese crib is defined as a dedicated "child bed," similar to a Western-style crib, that is enclosed with a headboard separating parents and children; the parents typically sleep immediately next to the child bed on another bed or futon. Alternatively, the child shares the adult bed or futon with the parents. The child bed is primarily used in infancy and is thought by Japanese parents to be particularly protective in the vulnerable neonatal period, because parents are concerned about the possibility of rolling over or otherwise hurting vulnerable neonates if they share the same bed.
The majority of infants (47.3%) slept on mattresses or futons; 80% of these coslept with their parents. Dedicated child beds were used exclusively by 37.3% of infants; of these, 52.4% usually coslept for part of the night. The rate of cosleeping among infants using child beds was significantly lower than in the adult-bed (P < .005) and futon (P < .001) groups. In the infant group only, children without SRNC were significantly more likely to sleep in separate, dedicated child beds than were those with SRNC (P < .02). The amount and pattern of cosleeping was examined in 3 categories: all night, during sleep onset and/or night waking, and during naptime. There were no between-category differences in SRNC.
SRNC and Sleep Patterns
Napping was not calculated for young infants, because pilot observations showed that parents could not identify stable patterns for children 2 to 3 months old. For older groups, the reported rate of daytime napping was 100% (n = 174) for toddlers and 72.3% (99 of 137) for preschoolers. Among toddlers, 2 children napped twice each day.
With the exception of children whose naptime was unstable or who napped twice per day, the average toddler nap began at 1:00 PM (n = 125), and the average preschooler nap began at 1:48 PM (n = 71). These data suggest that the naptime sleep phase was significantly delayed in preschoolers compared with toddlers (P < .001). In both groups, the incidence of irregular or late naptime was not related to SRNC status. The average nap duration for toddlers was 2.0 ± 0.6 hours (n = 167). Except for those with unstable nap durations and those who did not take naps, average nap duration was 1.8 ± 0.5 hours for preschoolers (n = 94). SRNC status was not related to nap duration; however, there was a significant decline in the duration of naps from toddler to preschool years (P < .005).
Parents were asked about their children's regularity of sleep onset. The average bedtime for toddlers and preschoolers was 9:36 PM (n = 162) and 9:30 PM (n = 125), respectively, ±1 hour. There were no significant differences in night sleep-onset time between these 2 groups. Among toddlers, significantly more (9.8% [11 of 112]) of those with but none of those without SRNC showed bedtime irregularity (P < .01). Among preschoolers, SRNC was not associated with bedtime irregularity. Late bedtime was not related to SRNC status in either group.
A "stable sleeper" was defined by the parental perception that a child usually slept the same amount each night. The average duration of nighttime sleep was not significantly different between toddlers (98.2% [168 of 171]; 9.8 ± 1.1 hour) and preschoolers (97.8% [133 of 136]; 10.1 ± 1.0 hours) who were stable sleepers. Preschoolers without SRNC were more likely to be stable sleepers that those with SRNC (P < .05).
A definition of "average" duration of sleep (9.5–10.5 hours) was also created based on the average across the toddler (9.8 hours) and preschool (10.1 hour) groups of stable sleepers in the sample. Preschoolers with SRNC (44.3% [35 of 79]) were less likely to have average sleep amounts, defined as sleep duration that was within the 9.5- to 10.5-hour range, than were preschoolers without SRNC (P < .5). There were no significant SRNC-related differences in stable sleepers and children with average sleep duration among toddlers.
SRNC and Other Factors
Family Structure and Child Care
Parent ratings of children's attractiveness as social partners, parent enjoyment of child rearing, and parent strictness in child rearing did not differ across age groups or with SRNC status. Among toddlers only, SRNC was significantly more common in nuclear families than in extended families (94.7% [107 of 113] vs 85.2% [52 of 61], respectively; P < .05). Also among toddlers only, 42.6% (48 of 113) of those without SRNC were rated as "cautious/wary" by parents. This trait, also translated as "carefulness," is viewed positively by parents and is thought to be associated with maturity. Toddlers without SRNC were significantly more likely to be so rated than were those with SRNC (P < .005). There were no SRNC-related differences in reactivity to unfamiliar situations.
Chronic Conditions
Among infants, toddlers, and preschoolers with SRNC, 35.7% (5 of 14), 81.3% (13 of 16), and 62.5% (10 of 16), respectively, suffered from chronic eczema. Across all groups, children with SRNC (12.4% [28 of 226]) were significantly more likely to have chronic eczema than were those without SRNC (7.1% [18 of 254]; P < .05).
The rate of SRNC with snoring was 44.4% (4 of 9) for infants, 70% (7 of 10) for toddlers, and 63.6% (7 of 11) for preschoolers. Across all groups, there were no differences in snoring between children with (7.9% [18 of 227]) and without (4.7% [12 of 254]) SRNC.
Of toddlers and preschoolers, 69.2% (9 of 13) and 80% (12 of 15), respectively, suffered from asthma. No cases of SRNC and asthma (or SRNC and bruxism) were reported among infants. There was no significant difference in the rate of SRNC with (10.8% [21 of 194]) and without (6.0% [7 of 116]) asthma. All toddlers (7 of 7) identified with bruxism had histories of SRNC, as did 90% (9 of 10) of preschoolers with SRNC. Overall, children with SRNC (8.2% [16 of 195]) had significantly more bruxism than did those without SRNC (0.9% [1 of 116]; P < .005).
Cosleeping and Child Bed Use in Infancy
Complex sleeping arrangements were seen only in infancy; almost all the toddlers and preschoolers coslept in or next to the adult bed. However, in infancy, cosleeping and child bed use were not exclusive categories (ie, partial cosleeping in the child bed was observed in some cases). Hence, 4 sleeping-arrangement categories were examined: (1) child bed without cosleeping; (2) child bed with cosleeping; (3) adult bed/futon or mixed pattern with cosleeping; and (4) adult bed/futon or mixed pattern without cosleeping. In the final analysis, stepwise, multiple logistic regression was used to examine sleeping arrangements, family structure, nonparental care, and chronic eczema as predictors of the development of SRNC during infancy. Only category 3 sleeping arrangements significantly predicted SRNC risk (P < .006). Hence, specifically during infancy, the combination of cosleeping and adult bed/futon or mixed-pattern sleeping was strongly associated with the development of SRNC.
DISCUSSION
Children who cosleep with their parents are known to have a higher prevalence of sleep problems, especially night waking.14–16,20 The results of our study suggest that the incidence of SRNC is related to sleeping arrangement, especially to patterns of cosleeping in infancy, as well as to individual characteristics of children and families. Lozoff and co-workers16 found that cosleeping per se was not associated with increased sleep problems in early childhood; night-waking levels were similar between cosleeping Japanese children and US children sleeping separately. In our study, Japanese parents and children coslept routinely at all the ages examined, but patterns among infants had subtle, important differences from those observed in the toddler and preschool groups.
In Western countries, sleep disorders have been found to be both common and persistent in early childhood. Of children with sleep problems at 8 months of age, 32% to 41% were observed to have continuing problems at 3 and 4 years of age.12,21 The results of our study suggest that the toddler age range is the most likely phase for development of SRNC. Parent reports of positive SRNC histories increased dramatically from infancy to toddlerhood but did not change between the toddler and preschool years. In fact, we found that SRNC had disappeared in 80% of children by the preschool years. We estimate that 60% of children had experienced SRNC by preschool age.
In agreement with previous work on the association of night waking and cosleeping, the phenomenon of SRNC seems to be related to traditional Japanese sleeping arrangements.20 In our study, the type of sleeping arrangement used in infancy was associated with SRNC. Specifically, infants who slept in dedicated child beds were much less likely to have SRNC than were those who had other arrangements. Infants whose parents endorsed cosleeping and who slept in or next to the parents' beds accounted for a threefold increase in risk for SRNC.
Regardless of the sleeping arrangement used, Japanese parents often cosleep on children's beds for the wake/sleep transition and sometimes after awakenings during the night,22 usually moving to their own beds after the children fall asleep. Anders et al23 reported that infants who were put into cribs already asleep at the beginning of the night were significantly more likely to develop the "signaling" pattern that alerts parents to night wakings by crying after spontaneous awakenings. Additionally, infants who developed these learned sleep-onset associations required parental intervention more often than did those who were put into their cribs awake at bedtime and allowed to fall asleep on their own. In an earlier study, we found that falling asleep in the parents' bed after night waking in infancy, particularly among breastfed infants, was associated with sleep disturbance and requests for cosleeping in early childhood.24 We suggest that SRNC may be governed by the same mechanism (ie, cosleeping in infancy encourages SRNC), whereas separate sleeping does not. Furthermore, parents are more likely to be aware of nighttime awakenings when infants sleep in close proximity to them.
The child bed was primarily used only during infancy. At toddlerhood, there was an apparent shift toward cosleeping, a pattern that, according to our data, was retained through the preschool years. Depending on family and child characteristics, it is typical to promote independent sleeping in another bed in the same or a different room at the beginning of elementary school. This may be related to SRNC incidence, because >80% of SRNC had disappeared by preschool age in our study.
In contrast to one frequently posed hypothesis,25,26 a well-controlled study found no association between colic and temperamental compromise.25,27 The developmental perspective is that neither colic nor SRNC is related to psychologic dysfunction or other outcome risk.1,25,28 In our study, the age range of the infant group was old for colic.29 Nonetheless, Clifford et al30 reported a prevalence of colic at 3 months of 6.4%, similar to the rate for SRNC (8.2%) that we observed.
Children with persistent sleep difficulties have been found to be more likely to have behavior problems, especially with tantrums and behavior management.13 In our study, toddlers who did not have SRNC were more likely to be reported to have the culturally desirable carefulness or cautious/wary trait than were those with SRNC, and toddlers who cried frequently during the day were more likely to have SRNC. Immature regulation of emotionality and/or anxiety, as represented by daytime temperamental characteristics, may be linked to immaturity of sleep-consolidation mechanisms.
SRNC symptoms seemed to be most frequent in toddlerhood. Interestingly, nonparental care in the toddler group, but not in other age groups, was associated with SRNC. It is important to note that in Japan, the reputations of day care centers, as in other industrialized countries, are variable, and day care quality was not assessed in our study. To date, there have been few reports concerning nonparental care and sleep problems.31 Nonetheless, day care was most commonly begun at the toddler age, and the associated change in routine from individual care may have increased nighttime anxiety at least initially.12 It is known that separation anxiety is related to night waking.32 In our study, toddlers, but not the other groups, showed SRNC significantly more commonly in nuclear families than in extended families, which suggests that toddlers may be more reactive to environmental changes or conditions.
We found that bruxism and atopic dermatitis were more common in toddlers and preschoolers with SRNC. With mild atopic dermatitis, awakening seems to be induced easily by itching.33 Atopic disorders in children have also been found to be associated with separation anxiety.34 Children older than our age groups with atopic dermatitis in clinical remission have been reported to have more frequent arousals and awakenings that are not related to scratching per se.33 Sleep bruxism is a relatively common childhood parasomnia and has been associated with high anxiety scores.35 Our findings support the hypothesis that frequent behavioral arousal and nighttime anxiety may contribute to SRNC. In addition, parents of children with chronic conditions such as eczema may have higher levels of concern and thus be more attentive to nighttime symptoms.
Western cultural practices of nonparental care are becoming increasingly common in Japan, as is the pressure on parents to meet daytime work demands. Coping with children who habitually wake, cry, and disturb their own and their parents' sleep is an important issue in child development and family health. This study was limited by being based on retrospective parent reports that were not objectively corroborated and by the severity and duration of SRNC not being ascertained. Nevertheless, the results suggest that infancy patterns of parent-child nighttime interactions and sleeping arrangements may provide guidance in the management and prevention of persistent night waking in early childhood.
ACKNOWLEDGMENTS
Special thanks to the staff of the Higashimurayama city public health centers for their generous cooperation; Alan Rosenwasser for comments on earlier versions of the manuscript; Patricia Hofmaster for consultation on biostatistical analyses; and Kathleen McAuliffe for help in preparation of the manuscript.
FOOTNOTES
Accepted Aug 5, 2004.
No conflict of interest declared.
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