What daily factors affect the sleep habits of Japanese toddlers?

Abstract

Study Objectives:

Good sleep, especially during early childhood, is important for development. In Japan, the mean nocturnal sleep duration of toddlers is < 10 hours, and even if toddlers slept for > 11 hours/day, as recommended by the National Sleep Foundation, some of them showed late bedtime and late wake-up time or took long naps. Therefore, we provisionally assumed the minimal sleep conditions for Japanese toddlers, named Nenne-criteria, such as bedtime before 10:00 pm, nocturnal sleep duration of ≥ 9 hours, and < 1 average time of awakening after sleep onset, and investigated the important factors for good sleep.

Methods:

We analyzed cross-sectional data from online surveys describing the sleep-related behaviors of 2,124 toddlers and their caregivers. We compared the daily schedules that affect sleep between the Nenne-criteria–meet group and the not-meet group.

Results:

The Nenne-criteria–meet group showed better daytime behaviors than the not-meet group. Structural equation modeling on daily schedules revealed that, to increase sleep pressure at the appropriate time, it is important to restrict media viewing, play outdoors in the morning, have an early nap ending time, avoid hyperarousal-inducing behaviors before bedtime, maintain daily schedules regularly, and decrease social jetlag.

Conclusions:

The Nenne-criteria are useful for screening Japanese toddlers who require intervention for sleep hygiene. To improve toddlers’ sleep, it is important not only to guide the ideal bedtime but also to provide tips for improving daily schedules and to avoid suboptimal sleep-related behaviors.

Citation:

Murata E, Yoshizaki A, Fujisawa TX, Tachibana M, Taniike M, Mohri I. What daily factors affect the sleep habits of Japanese toddlers? J Clin Sleep Med. 2023;19(6):1089–1101.

BRIEF SUMMARY

Current Knowledge/Study Rationale: The National Sleep Foundation recommended that toddlers sleep ≥ 11 hours/day; however, many Japanese toddlers sleep < 10 hours at night, or even if they sleep ≥ 11 hours, some of them show late bedtime and wake-up time or take a long nap. There is a need for minimal criteria for intervention for Japanese toddlers’ sleep.

Study Impact: For a good sleep schedule, it is important to restrict media viewing, play outdoors in the morning, end nap time early, avoid hyperarousal-inducing behaviors before bedtime, maintain daily schedules regularly, and decrease social jetlag. It is important to inform parents about the ideal bedtime and to provide tips to improve daily schedules and to avoid conditioning.

INTRODUCTION

Good sleep, especially during early childhood, is important for physical and cognitive development.¹ Children with longer sleep durations were better at emotional and behavioral regulation and cognitive performance.^2,3 Good sleep health is characterized by the absence of sleep disorders and appropriate sleep habits, including early bedtime, adequate sleep duration, high sleep efficiency, sustained alertness during waking hours, and self-reported satisfaction.⁴ With good sleep habits, toddlers wake up spontaneously (eg, without alarm clocks or repeated parental reminders) in the morning at the desired time,³ eat breakfast well,⁵ and have good temperament and activity levels during the day.⁶

Toddlers are recommended to sleep at least 11 hours/day according to the National Sleep Foundation (NSF).⁷ In an international, cross-cultural comparison with a large sample of children aged from 0 to 36 months, Japanese children had the shortest total sleep duration.⁸ We have previously reported that a bedtime after 10:00 pm leads to short nocturnal sleep duration in children attending nursery school or kindergarten and late get-up time or long nap duration in children who stay at home.⁹ The above previous study also reported that children from predominantly Asian countries had later bedtimes, later wake-up times, and less total sleep duration than predominantly Western countries.⁸ Even in Western countries, later bedtime and later wake-up time were reportedly more common among families living in poverty.¹⁰

In organized sleep regulation, 2 processes play a dominant role: a sleep-dependent process (process S) and a sleep-independent circadian process (process C).¹¹ Process S is composed of homeostatic sleep pressure, which increases during waking as an exponential saturating function and decreases exponentially during sleep.¹² Process C influences the internal organization of sleep and timing and the duration of daily sleep–wake cycles. It also governs predictable patterns of alertness throughout the 24-hour day.³

Considering external factors, it is important to avoid screen use including television (TV), tablets, etc, around bedtime.¹³ Moreover, bedtime-related conditioning—for example, breastfeeding—has been reported to be associated with toddlers’ insomnia,¹⁴ and contrarily, bedtime adaptive activities, such as feeding, bathing/brushing teeth, and reading, are associated with fewer sleep problems.¹⁵ In addition, an irregular daily schedule, which has been reported to be associated with problematic daytime behaviors,¹⁶ was observed in 53% of Japanese children aged 1−6 years.¹⁷

Children’s sleep problems lead to parenting stress, parental mental health problems, and marital dissatisfaction.¹⁸ However, children’s sleep regulation is often challenging for parents¹⁹ due to multiple factors that affect children’s sleep, including nap schedule, screen time and timing,^9,20,21 suppertime,¹⁷ the presence of older siblings,^22,23 and the amount of physical activity.²⁴ To give parents repeated customized advice for improving their children’s sleep, we developed a mobile phone application named Nenne Navi (Osaka University, Osaka, Japan) as a parental guidance tool²⁵ (“Nenne” is a Japanese word that means “sleep” used in baby talk). We provisionally assumed 3 minimal sleep conditions named the “Nenne-criteria,” which included bedtime before 10:00 pm, 9 or more hours of sleep duration at night, and less than 1 average time of awakening after sleep onset; and we intervened in the participants who did not meet the Nenne-criteria via the Nenne Navi in our intervention study²⁶. The bedtime and sleep durations in the Nenne-criteria are not sufficiently satisfactory for a toddler’s sleep compared with Western standards. In fact, in the United States, approximately 10% of toddlers go to sleep after 10:00 pm²⁷; however, in Japan, approximately 40% of toddlers often go to sleep after 10:00 pm⁹ Even among children whose sleep conditions fall within the NSF criteria, many sleep late and get up late. Furthermore, in New Zealand, less than 10% of children aged 3−4 years had social jetlag, circadian misalignment that occurs when people shift their sleep schedules from weekdays to weekends²⁸; however, in Japan, 53% of the 1−6-year-old children had social jetlag.¹⁷ Children’s sleep is largely affected by their caregivers’ lifestyle⁹; thus, in Japan, public awareness activities for sleep hygiene are insufficient and customized instructions for children with poor sleep habits are needed. With regard to sleep time, in Japan, 50.4% of toddlers attended either nursery school or daycare²⁹ and usually napped for 2 hours at most; therefore, we considered 9 or more hours of sleep at night to be the minimum requirement for securing a total sleep duration of 11 hours/day as recommended by the NSF.⁷ Furthermore, awakening after sleep onset, which has been reported to affect the health of children³⁰ and the mental health of mothers,^31,32 is a target for sleep intervention. A systematic review reported that children aged between 1 and 2 years awakened 0.7 times per night, on average³³; therefore, the condition that the average times of awakening after sleep onset is less than 1 per night was adapted to our criterion.

In our aforementioned study, we have demonstrated the effect of interventions for toddlers with sleep problems (in preparation). Thus, we believe it is meaningful to clarify the factors of effective intervention to improve toddlers’ sleep habits in Japan by assessing the community sleep habits based on the Nenne-criteria. In this study, by accumulating a large sample of sleep-related habits of toddlers, we examined the adherence to the Nenne-criteria and clarified the factors that affect sleep problems in Japanese toddlers. Furthermore, using structural equation modeling (SEM), we proposed a model for assessing the relationship between daily routines and sleep-related schedules in Japanese toddlers based on hypotheses from previous studies. As mentioned above, sleep regulation is organized by process C and process S.^3,11,12 These processes are biological systems inherent in humans from before the introduction of digital devices. With regard to the influence of digital devices on sleep, media viewing before bedtime and/or for long hours leads to sleep problems.¹³ In short, hyperarousal behavior before bedtime may decrease sleep pressure. Breastfeeding in response to nocturnal awakening is associated with increased sleep fragmentation²²; thus, such conditioning may lead to more frequent nocturnal awakenings. It is possible to provide parents with more evidence-based advice for Japanese toddlers if the factors closely related to their sleep habits are clarified.

METHODS

Study design and participants

We compared the cross-sectional data of online surveys describing the sleep behavior of toddlers and caregivers. The data on 4,912 toddlers were collected from March 3 to March 10, 2019. We excluded toddlers with missing data concerning daily routines and sleep-related schedules (n = 2,700) and analyzed the available data on the toddlers for at least 4 consecutive days to ascertain their nocturnal sleep patterns. Consequently, data on 2,124 participants were used in the final analysis. Recruited toddlers were divided into 2 groups: those who met the Nenne-criteria (Nenne-meet group) and those who did not (Nenne-not-meet group).

The participants of this study were fluent in Japanese and lived in Japan. The total population of Japan comprises 97.8% Japanese individuals³⁴; thus, we ascertained that the participants of this study were predominantly Japanese.

Ethics approval and consent to participate

This study was approved by the Ethics Committee of the Pedagogy course of the Graduate School of Human Sciences, Osaka University (approval no. 18058), on February 20, 2019. All study procedures were conducted in accordance with the ethical standards of the Declaration of Helsinki. As part of obtaining participant consent, a research briefing was presented online to the (potential) participants to brief them on the research purpose and general outline of the study. The participants were deemed to have consented to the study upon clicking on the provided “Agree” button. If the participants wanted to resign from participating in this research, they were freely able to drop out without contacting us. The caregiver of each participant provided informed consent on our website and received a coupon for reward points valued at ¥500 upon completion of the survey.

Procedure and questionnaire

We conducted an online survey of caregivers of toddlers aged 18−30 months through a research company (INTAGE, Inc, Tokyo, Japan), which had many survey panelists for various research conducted by corporations, research institutes, etc. In Japan, toddlers in this age group either attend nursery school or stay at home. As attending nursery school affects the toddler’s sleep habituation,⁹ caregivers were asked to answer where the toddler spends time during the day. We only collected data regarding sleep-related habits and did not gather data regarding personal information, such as name, date of birth, and residence. For 8 consecutive days, we asked caregivers to report their toddlers’ daily schedule (such as wake-up time, bedtime, nap time, dinner time, bath time), activities affecting sleep (daytime physical activity, TV viewing, etc), and daytime mood; these questions (Tables S1–S6 ^{(476.2KB, pdf)} in the supplemental material) are identical to those used in the Nenne Navi application.²⁵ Based on the information obtained from the responses to the abovementioned questions on Nenne Navi, we provided customized advice for each caregiver.

Although there is no consensus on the definition of enough sleep for each toddler despite research across various fields, including electrophysiological and autonomic nerve studies, as seen in previous studies, sufficient sleep leads to good behavior in a child.^2,3,5,6,18 The way in which the child wakes up, mood at the time of waking up, appetite for breakfast, and mood during the day were associated with the child’s sleep quantity and/or quality.^2,3,5,6,18 Therefore, we selected “waking up independently,” “being in a good mood in the morning,” “good appetite at breakfast,” and “being in a good mood during the day” as behavioral indicators of adequate sleep. Caregivers were asked to answer the following questions: “How did your child wake up this morning?” on a 3-point scale (woke up independently/woke up immediately when being woken up by others/was necessary to be woken up by others repeatedly), “How was your child’s mood when he/she woke up this morning?” on a 5-point scale (good/relatively good/neutral/relatively bad/bad), “Did your child eat breakfast well this morning?” on a 3-point scale (ate breakfast well/did not eat breakfast well/barely ate breakfast), and “How was your child’s mood today?” on a 5-point scale (good/relatively good/neutral/relatively bad/bad). The answers “woke up independently,” “good,” “relatively good,” and “ate breakfast well” accounted for desired behavior. We divided the toddlers into 2 groups: those who showed the desired behavior for all 4 questions for more than 75% of the days and those who did not, and compared those rates between the Nenne-criteria and NSF recommendation.⁷

The following question on the times of nocturnal awakenings was used: “Last night, did the child wake up and stay awake or cry during the night for five minutes or longer?” “Please enter ‘0 times’ if the child woke up or cried for less than five minutes.” The reason for setting 5 minutes as a border of the definition for nocturnal awakening was based on a review of 30 studies using actigraphy that reported the need for more than 5 minutes of arousal for discriminating nocturnal awakening.³⁵

In addition to media-viewing time, the end time of TV viewing was asked if the child watched TV after 4:00 pm, as mentioned above by a previous study because TV viewing in the evening or night tends to affect sleep.²⁰ The sum of media-viewing time and the end time of total media viewing after 4:00 pm were used for analysis.

We calculated the percentage of toddlers who breastfed at night among the toddlers who breastfed daily. Since the presence of older siblings affects sleep habits,^22,23 we also examined the correlations between the number of older siblings and the toddler’s sleep habits. The sleep schedule of children was affected by their family background and their parents’ daily schedule.^15,22,36 Thus, we asked about the caregivers’ sleep habits as well.

Statistical analysis

The t test or chi-square test was used to compare the demographic data of the Nenne-meet and Nenne-not-meet groups, and the Mann−Whitney U test was used to compare temporal values between the 2 groups. The chi-square test was used for the comparison of the toddlers’ daytime behaviors indicating adequate sleep, such as “the child woke up independently,” “good or relatively good mood when waking up,” “having a good appetite for the breakfast,” and “good or relatively good mood throughout the day,” between the Nenne-criteria and sleep duration recommended by the NSF.⁷

In addition, multiple regression analysis was performed to examine the effect of confounding factors in both groups. To evaluate the irregularity of the toddlers’ sleep rhythms, standard deviations (SDs) were calculated for variables that had been recorded continuously for more than 4 days, and the SD was used as an index of irregularity. Social jetlag was calculated as differences in the midpoint of weekend nocturnal sleep duration and the midpoint of weekday nocturnal sleep duration as proposed by Wittmann et al.³⁷ In addition, the correlations between the number of older siblings and the toddler’s sleep habits were examined using the Pearson correlation test.

The SEM was used to elucidate what factors are important for good sleep habits in Japanese toddlers based on the following hypothesis, which was obtained by analyzing confirmatory factors to ascertain that the hypothesis was either accepted or rejected. We considered 4 latent variables: sleep pressure, which roughly corresponds to process S, as proposed by Borbély¹¹; irregularity of daily schedule, which impedes process C¹¹; activities near bedtime, such as media viewing, which increase arousal level; and bedtime-related conditioning, such as breastfeeding. The evidence behind the hypothesis is stated in the Introduction section.^3,11–13,22 A path diagram was created using the data on Nenne-not-meet toddlers (do not match the Nenne-criteria: there are days when the bedtime is after 10:00 pm, or there is a nocturnal sleep duration of < 9 hours; another criterion is > 1 average time of awakening after sleep onset, n = 1,234), and the goodness-of-fit index, adjusted goodness-of-fit index, comparative fit index, and root mean square error of approximation were used to assess the degree of fit. Since this model cannot be created if there are missing values, we excluded 2 toddlers: 1 due to missing data on the start and end times of naps and the other due to missing data on the end time of bathing.

SPSS version 26 (IBM Japan Ltd, Tokyo, Japan) was used to analyze the data, and AMOS version 26 (IBM Japan Ltd) was used to create the model.

RESULTS

Participants’ demographic characteristics

Table 1 shows the demographic information for the participants. Almost all of the toddlers slept in bedrooms, slept with their caregivers (Table 1), and did not have underlying disorders (Table S7 ^{(476.2KB, pdf)} ).

Table 1.

Participants’ demographic characteristics.

Variable	Value (n = 2,124)
Toddler’s age, mean ± SD, mo	24.2 ± 3.8
Toddler’s sex (%)
Male (n = 1,075)	50.6
Female (n = 1,049)	49.4
Attending nursery school ≥ 3 days/wk (%)	33.9
Sleeping place (%)
Bedroom	92.8
Living room	5.7
Other	1.5
Sleeping style (%)
Sleeping with caregivers	86.2
Sleeping alone	13.0
Other	0.8
Number of siblings, mean ± SD	1.3 ± 0.6
Informant (%)
Mother	78.5
Father	21.4
Grandfather	0.1
Mother’s age, mean ± SD (n = 2,105), y	34.0 ± 4.7
Father’s age, mean ± SD (n = 2,034), y	36.4 ± 5.9
Mother’s educational level (%)
Junior high school	2.0
High school	21.0
Professional training college/junior college	34.7
University graduate/completed graduate school	38.7
Other/not answered	3.6
Father’s educational level (%)
Junior high school	3.1
High school	24.6
Professional training college/junior college	15.6
University graduate/completed graduate school	53.7
Other/not answered	3.0
Working mothers (%)	40.0
Working fathers (%)	98.4
Annual income (including taxes) (%)
≤ $19,999	2.3
$20,000–$39,999	20.0
$40,000–$59,999	31.2
$60,000–$79,999	17.3
$80,000–$99,999	6.7
≥ $100,000	5.6
Do not know/no answer	16.9
Single mother (%)	2.8
Single father (%)	0

SD = standard deviation.

The most common educational level of the parents was university graduate/completed graduate school, which is slightly higher than the data on the same generation in Japan (females 34.8%, males 39.9%).³⁴ Major households had an annual income of $40,000−$59,999 including taxes. The median and average household income reported by the Ministry of Health, Labor, and Welfare was $43,700, and $55,230, respectively.³⁸ Therefore, the households in this study could be considered to represent the average Japanese household. In addition, there were 60 single mothers (2.8%) and no single fathers, which did not largely deviate from the previous report of 1.2% single mothers and 0.1% single fathers reported by the Ministry of Health, Labor, and Welfare.³⁸

Characteristics of the Nenne-meet and Nenne-not-meet groups

Table 2 shows the demographic information of both groups. The toddlers’ ages, percentage of those attending nursery school, number of siblings, mother’s age, father’s age, mother’s educational level, and percentage of working mothers were significantly different between the 2 groups; however, the effect sizes were extremely small, excluding the percentage of those attending nursery school.

Table 2.

Demographic characteristics of the “Nenne-meet” and “Nenne-not-meet” groups.

Variable	Nenne-meet (n = 752)	Nenne-not-meet (n = 1,372)	P	Effect Size
Age, mo	24.0 ± 3.8	24.3 ± 3.8	< .05a	.05c
Sex, n (%)
Male	402 (53.5%)	673 (49.1%)	.052b	.04d
Female	350 (46.5%)	699 (50.9%)
Attending nursery school, n (%)
≥ 3 days per week	202 (26.9%)	517 (37.7%)	< .001b	.11d
< 3 days per week	550 (73.1%)	855 (62.3%)
Number of siblings	1.3 ± 0.5	1.4 ± 0.7	< .005a	.10c
Number of children with older siblings	369	653	.515b	−.01d
Mother’s age, y	33.6 ± 4.4	34.2 ± 4.8	< .01a	.07c
Father’s age, y	36.1 ± 5.6	36.6 ± 6.0	< .05a	.05c
Mother’s educational level, n (%)
Junior high school	12 (1.6%)	31 (2.4%)	< .05b	.071d
High school	145 (19.8%)	302 (22.9%)
Professional training college/junior college	248 (33.9%)	489 (37.1%)
University graduate/completed graduate school	326 (44.6%)	495 (37.6%)
Father’s educational level, n (%)
Junior high school	17 (2.4%)	47 (3.6%)	.092b	.057d
High school	164 (23.1%)	343 (26.5%)
Professional training college/junior college	113 (15.9%)	210 (16.2%)
University graduate/completed graduate school	416 (58.6%)	693 (53.6%)
Parent’s employment status, n (%)
Working mother	264 (35.8%)	586 (43.9%)	< .001b	−.08d
Stay-at-home mother	473 (64.2%)	750 (56.1%)
Working father	717 (99.3%)	1315 (99.8%)	.108b	−.04d
Stay-at-home father	5 (0.7%)	3 (0.2%)
Annual income (including taxes), n (%)
≤ $19,999	17 (2.7%)	32 (2.8%)	.908b	.030d
$20,000–$39,999	159 (25.2%)	265 (23.4%)
$40,000–$59,999	235 (37.3%)	428 (37.7%)
$60,000–$79,999	123 (19.5%)	244 (21.5%)
$80,000–$99,999	53 (8.4%)	90 (7.9%)
≥ $100,000	43 (6.8%)	75 (6.6%)
Single mother, n	19	41	.539b	.01d

Values are presented as mean ± SD unless otherwise indicated. Mother’s age: Nenne-meet, n = 748; Nenne-not-meet, n = 1,357. Father’s age: Nenne-meet, n = 720; Nenne-not-meet, n = 1,314. Mother’s educational level: Nenne-meet, n = 731; Nenne-not-meet, n = 1,317. Father’s educational level: Nenne-meet, n = 710; Nenne-not-meet, n = 1,293. Mother’s employment status: Nenne-meet, n = 737; Nenne-not-meet, n = 1,336. Father’s employment status: Nenne-meet, n = 722; Nenne-not-meet, n = 1,318. Annual income: Nenne-meet, n = 630; Nenne-not-meet, n = 1,134. ^a t test. ^b chi-square test. ^c r. ^d φ. SD = standard deviation.

Difference in the daytime behavior between toddlers who met the Nenne-criteria and those who met the NSF recommendation

As shown in Table 3, we compared the percentage of toddlers who showed desired behaviors between the Nenne-criteria and the NSF recommendation.⁷ The percentage of toddlers who showed good daytime behavior was significantly higher in the Nenne-meet group than in the Nenne-not-meet group (P < .001); however, there was no significant difference between the groups divided according to the NSF criteria (P = .123).

Table 3.

Comparison of the toddlers’ daytime behavior by criteria.

“How did your child wake up this morning? —Woke up independently” “How was your child’s mood when he/she woke up this morning? — Good/relatively good” “Did your child eat breakfast well this morning? —Ate breakfast well” “How was your child’s mood today? —Good/relatively good”*			Others	Chi-square test P	Effect Size φ
Nenne criteria	Nenne-meet (n = 752, 35.4%)	236 (31.4%)	516 (68.6%)	< .001	.110
	Nenne-not-meet (n = 1,372, 64.6%)	294 (21.4%)	1,078 (78.6%)
Sleep duration recommended by National Sleep Foundation	≥ 11 hours (n = 1,602, 75.4%)	413 (25.8%)	1,189 (74.2%)	.123	.123
	< 11 hours (n = 522, 24.6%)	117 (22.4%)	405 (77.6%)

^*Indicates toddlers in whom the desired response occurred on ≥ 75% of days for that item.

Comparison of sleep and sleep-related habits and daytime behavior

Table 4 shows the mean and median values of the data associated with sleep and sleep-related habits and activity of the 2 groups. The Nenne-meet group had an earlier median daily schedule time than did the Nenne-not-meet group. The Nenne-meet group spent less time before bedtime after bathing. In the Nenne-not-meet group, the irregularity of each time was broader than in the Nenne-meet group. In the Nenne-meet group, social jetlag was also less than in the Nenne-not-meet group. In the Nenne-meet group, not only toddlers but caregivers also had earlier wake-up times and bedtimes, longer nocturnal sleep durations, and smaller irregularities in these variables. In addition, toddlers in the Nenne-meet group had a significantly lower percentage of breastfeeding at awakening after sleep onset.

Table 4.

Comparison of sleep and sleep-related habits and activity between the “Nenne-meet” and “Nenne-not-meet” groups.

Sleep Habits and Daytime Behavior	Nenne-meet (n = 752)			Nenne-not-meet (n = 1,372)			P a	P b	Effect Size r
	Mean	Median	Interquartile Range	Mean	Median	Interquartile Range
Wake-up time	7:09 am	7:08 am	6:48–7:33 am	7:24 am	7:19 am	6:49–7:51 am	< .001	< .001	.12
Irregularity of wake-up time (min)	22.4	20.9	14.5–28.7	30.2	27.8	19.8–37.7	< .001	< .001	.26
Bedtime	8:30 pm	8:35 pm	8:06–8:59 pm	9:18 pm	9:17 pm	8:52–9:43 pm	< .001	< .001	.51
Irregularity of bedtime (min)	19.8	15.9	10.5–26.2	29.1	25.5	16.0–36.8	< .001	< .001	.28
Sleep-onset latency (min)	19.5	17.9	8.6–27.1	23.1	20.0	11.4–30.7	< .001	< .001	.10
Irregularity of sleep-onset latency (min)	10.6	9.9	7.0–14.1	13.5	11.3	8.7–16.8	< .001	< .001	.13
Nocturnal sleep duration	10 h, 18 min	10 h, 14 min	9 h, 54 min–10 h, 39 min	9 h, 39 min	9 h, 36 min	9 h, 12 min–10 h, 4 min	< .001	< .001	−.46
Irregularity of nocturnal sleep duration (min)	30.1	27.6	19.2–37.1	42.8	38.7	28.4–52.5	< .001	< .001	.34
Social jetlag (min)	17.5	14.0	6.0–26.0	24.4	18.0	9.0–34.0	< .001	< .001	.16
Number of times awakening after sleep onset	0.2	0.1	0–0.3	0.5	0.1	0–0.6	< .001	< .001	.18
Breastfeeding at night (%)	4.4	0*	0–0*	25.0	0*	0–50.0	< .001	< .001	.26
Nap start time	1:02 pm	1:00 pm	12:24–1:39 pm	1:14 pm	1:04 pm	12:33–1:52 pm	< .005	< .001	.07
Irregularity of nap start time (min)	52.7	48.3	27.5–74.9	59.7	53.7	32.2–81.8	< .001	< .001	.09
Nap end time	2:48 pm	2:49 pm	2:14–3:23 pm	3:13 pm	3:04 pm	2:34–3:49 pm	< .001	< .001	.17
Irregularity of nap end time (min)	55.3	51.2	27.5–76.8	63.3	59.5	32.2–88.5	< .001	< .001	.10
Nap duration	1 h, 24 min	1 h, 27 min	1 h, 1 min–1 h, 49 min	1 h, 37 min	1 h, 41 min	1 h, 14 min–2 h, 1 min	< .001	< .001	.17
Irregularity of nap duration (min)	31.4	29.8	19.8–42.0	35.9	34.3	22.8–46.5	< .001	< .005	.11
Total sleep duration	11 h, 43 min	11 h, 44 min	11 h, 18 min–12 h, 5 min	11 h, 17 min	11 h, 17 min	10 h, 52 min–11 h, 44 min	< .001	< .001	−.30
Irregularity of total sleep duration (min)	40.4	37.8	27.7–50.8	51.1	46.9	34.8–62.8	< .001	< .001	.24
TV-viewing time	1 h, 52 min	1 h, 43 min	56 min–2 h, 28 min	1 h, 53 min	1 h, 39 min	1 h–2 h, 34 min	.678	.085	.01
End time of TV viewing	7:39 pm	7:50 pm	6:56–8:30 pm	8:32 pm	8:43 pm	7:45–9:20 pm	< .001	< .001	.34
Smartphone-viewing time (min)	9.13	0*	0–7.0	17.8	1.0	0–17.0	< .001	< .005	.12
End time of smartphone viewing	4:59 pm	5:40 pm	3:00–7:17 pm	5:58 pm	6:43 pm	4:17–8:00 pm	< .001	< .005	.16
Total media-viewing time	2 h, 1 min	1 h, 51 min	1 h–2 h, 39 min	2 h, 11 min	1 h, 59 min	1 h, 11 min–2 h, 50 min	< .05	< .005	.05
End of total media-viewing time	7:40 pm	7:51 pm	7:00–8:30 pm	8:33 pm	8:43 pm	7:47–9:21 pm	< .001	< .001	.34
Play outside for 30 minutes or more in the morning (%)	32.7	29.0	14.0–57.0	27.1	14.0	0–43.0	< .001	.068	−.11
Dinner end time	6:36 pm	6:34 pm	6:13–7:00 pm	6:58 pm	7:00 pm	6:30–7:24 pm	< .001	< .001	.27
Irregularity of dinner end time (min)	22.2	21.0	13.6–28.8	26.7	25.0	16.0–34.7	< .001	< .001	.15
Bathing end time	6:51 pm	7:05 pm	6:09–7:39 pm	7:27 pm	7:39 pm	6:50–8:13 pm	< .001	< .001	.26
Irregularity of bathing end time (min)	29.8	22.5	13.6–39.3	36.0	28.7	17.3–49.0	< .001	< .001	.14
Time between bathing and bedtime	1 h, 39 min	1 h, 29 min	57 min–2 h, 9 min	1 h, 52 min	1 h, 38 min	1 h, 10 min–2 h, 23 min	< .001	< .001	.12
Irregularity of time between bathing and bedtime (min)	27.3	20.8	12.2–35.6	34.6	28.0	16.4–45.9	< .001	< .001	.17
Going out after 7:00 p.m. (%)	3.5	0*	0–0*	7.7	0*	0–14.0	< .001	< .001	.18
Caregiver wake-up time	6:43 am	6:45 am	6:19–7:09 am	6:49 am	6:49 am	6:16–7:19 am	< .05	< .001	.05
Irregularity of caregiver wake-up time (min)	29.2	25.7	16.9–38.2	36.6	32.3	20.9–47.0	< .001	< .001	.17
Caregiver bedtime	10:43 pm	10:45 pm	9:43–11:36 pm	10:58 pm	10:54 pm	10:04–11:44 pm	< .001	< .005	.08
Irregularity of caregiver bedtime (min)	43.2	36.4	20.0–59.5	43.9	37.8	22.7–57.6	.260	.969	.02
Caregiver sleep-onset latency (min)	29.8	21.3	12.5–38.8	32.2	23.1	12.5–44.4	.061	< .05	.04
Irregularity of caregiver sleep-onset latency (min)	14.9	10.8	6.6–20.2	15.9	11.3	6.6–22.3	.242	< .01	.03
Caregiver nocturnal sleep duration	7 h, 29 min	7 h, 31 min	6 h, 46 min–8 h, 16 min	7 h, 18 min	7 h, 22 min	6 h, 36 min–8 h, 5 min	< .005	< .05	−.07
Irregularity of caregiver nocturnal sleep duration (min)	52.9	48.0	32.2–68.9	56.8	51.5	35.7–71.4	< .005	< .05	.07
Caregiver social jetlag (min)	27.3	19.0	9.0–38.0	32.7	25.0	11.0–47.0	< .001	< .005	.09

The percentages of “Breastfeeding at night” were calculated from the population who answered “breastfeeding” in the demographic survey, and who answered “breastfed at night,” “breastfed before 30 minutes of bedtime,” or “breastfed in the bed” in daily question, n = 408. ^a Mann–Whitney U test. ^b Multiple regression analysis, adjusted for toddler’s age, nursery school attendance, number of siblings, mother’s age, father’s age, mother’s educational level, and mother’s employment status. * Most of the data were 0. TV = television.

With regard to media viewing, the median TV-viewing time was not different between the 2 groups; however, the end time of TV viewing, smartphone-viewing time, and end time of smartphone viewing were significantly shorter and earlier in the Nenne-meet group. Furthermore, total media-viewing time and the end time of total media-viewing time were also significantly shorter and earlier in the Nenne-meet group.

In the Nenne-meet group, a higher proportion of toddlers played outside for 30 minutes or more in the morning and less often went out after 7:00 pm than in the Nenne-not-meet group.

Table 5 shows the activities before bedtime of the 2 groups. In terms of activities 30 minutes before bedtime, the Nenne-meet group less frequently spent time with media and more often spent time reading picture books. Among in-bed activity, reading picture books was significantly higher in the Nenne-meet group.

Table 5.

Comparison of the activities before bedtime between the “Nenne-meet” and “Nenne-not-meet” groups.

Activity Before Bedtime	Nenne-meet (n = 717)			Nenne-not-meet (n = 1,300)			P a	P b	Effect Size r
	Mean	Median	Interquartile Range	Mean	Median	Interquartile Range
Activities 30 minutes before bedtime (%)
Media viewing	13.1	0*	0–14.0	19.6	0*	0–29.0	< .001	< .005	.12
Playing with toys	44.2	43.0	14.0–71.0	44.7	43.0	14.0–71.0	.618	.770	.01
Physical play	14.3	0*	0–14.0	12.6	0*	0–14.0	.659	.864	−.01
Breastfeeding	34.1	14.0	0–57.0	42.4	29.0	0–86.0	.052	.111	.10
Bathing	24.4	0*	0–43.0	20.5	0*	0–29.0	.054	< .05	−.04
Brushing teeth	66.3	86.0	29.0–100	66.2	86.0	29.0–100	.456	.636	−.02
Reading picture books	29.9	14.0	0–57.0	25.5	14.0	0–43.0	< .005	.090	−.07
In-bed activities (%)
Media viewing	2.4	0*	0–0*	4.6	0*	0–0*	< .001	.453	.08
Playing with toys	5.7	0*	0–0*	7.3	0*	0–0*	< .05	.308	.05
Listening to music	2.7	0*	0–0*	2.6	0*	0–0*	.392	.161	.02
Bedtime theater	1.4	0*	0–0*	2.0	0*	0–0*	.123	.344	.03
Breastfeeding	46.1	29.0	0–100	49.0	57.0	0–89.5	.615	.709	.03
Only reading picture books	14.0	0*	0–14.0	10.8	0*	0–0*	< .05	< .05	−.05

The percentages of “The ways to spend time before 30 minutes of going to bed—breastfeeding,” and “The ways to spend time in the bed—breastfeeding” were calculated from the population who answered “breastfeeding” in the demographic survey and who answered “breastfed at night,” “breastfed before 30 minutes of bedtime,” or “breastfed in the bed” in daily question; n = 408. ^a Mann–Whitney U test. ^b Multiple regression analysis, adjusted for toddler’s age, nursery school attendance, number of siblings, mother’s age, father’s age, mother’s educational level, and mother’s employment status. * Most of the data were 0.

The results shown in Table 4 and Table 5 were statistically significant despite controlling for confounding factors, such as toddler’s age, nursery school attendance, number of siblings, mother’s age, father’s age, mother’s educational level, and mother’s employment status. On the contrary, after controlling for these confounding factors, there were no significant differences in the proportion of toddlers who played outside for 30 minutes, read picture books before bedtime, used media in bed, and played with toys in bed.

Additionally, the results in Table 4 and Table 5 were also statistically significant even after controlling for confounding factors, such as “with or without asthma and atopic dermatitis,” which is a factor that may affect sleep,³ or “with or without primary illness.”

Clarifying what daily factor is important for the good sleep habituation in Japanese toddlers using SEM

To examine the influence of the time of daily schedules and activities to sleep, we created the path diagram (Figure 1). It was an ideal SEM (goodness-of-fit index = 0.962, adjusted goodness-of-fit index = 0.944, comparative fit index = 0.924, and root mean square error of approximation = 0.052). “Sleep pressure,” which is equivalent to process S, was affected by the toddlers’ wake-up time, etc. We also attempted to create a model including other variables that possibly affect sleep pressure, such as the end times of dinner and bathing and the time between bathing and bedtime. However, some variables did not show significant differences, and multicollinearity was suggested; therefore, the model was deemed nonoptimal. “Irregularity of sleep,” which was connected with process C, was significantly affected by the irregularities of the toddler’s wake-up time, etc, and social jetlag. For “Hyperarousal,” which was the influence of the hyperarousal behavior before bedtime, the end time of the total media viewing had a strong influence. We also attempted to create a model that included other variables that were possible causes of hyperarousal, such as going out after 7:00 pm and activity 30 minutes before going to bed. However, some variables did not show significant differences, and reduced the model fitness.

Figure 1. The sleep habit model to clarifying the daily routine is important for the good sleep habituation in Japanese toddlers.

Squares indicate the “observed variables,” which are the input data, such as the toddler’s wake-up time. Circles with words indicate the “latent variable,” which is the construct that we are unable to directly observe. Circles with “e” and a number are “error variables”—namely, the variables of the other factors of the data that need to be analyzed. Variables that affect “sleep pressure,” “hyperarousal,” “irregularity of schedule,” and “conditioning” and their relationships are shown. All of the standardized estimates show significant differences at **P < 0.001, *P < 0.01.

Breastfeeding at awakening after sleep onset had the largest effect, and breastfeeding in bed also had some effect on “Conditioning,” which was connected with behavioral insomnia in childhood. “Conditioning” largely affected the number of times that awakening occurred after sleep onset. The relationships among the latent variables showed that “Hyperarousal” and “Irregularity of sleep” had a considerable effect on “Sleep pressure.”

DISCUSSION

The daytime behavior for good sleep in Japanese toddlers

As shown in Table 3, toddlers in the Nenne-meet group had significantly better daytime behavior than those in the Nenne-not-meet group. In this study, 75% of the toddlers slept for 11 or more hours; however, many Japanese toddlers go to bed late^8,9,17 and have short nocturnal sleep durations,²¹ as also shown in this study (39.3% of toddlers in this study went to bed after 10:00 pm, and 47.0% of toddlers slept < 9 hours during the night). Furthermore, 12.9% of toddlers wake up 1 or more times at night after sleep onset, causing distress to caregivers and making child-rearing difficult, as reported previously.^31,32 Although a clear consensus on the definition of good sleep does not exist, it was reported that children who have enough sleep show good daytime behaviors.^2,3,5,6,18 We selected the 4 behaviors as behavioral indices of adequate sleep as shown in Table 3; consequently, it indicated that the Nenne-criteria are more appropriate for selecting the Japanese toddlers who need intervention for good sleep habits than the criterion of more than 11 hours of total sleep duration.

Comparison between the Nenne-meet and Nenne-not-meet groups

As shown in Table 4, toddlers in the Nenne-meet group had significantly earlier and more regular daily schedules than those in the Nenne-not-meet group. This is consistent with previous studies showing that toddlers with earlier bedtimes have better sleep,^3–5 and that families that go to bed early and wake up early on both weekdays and weekends have regular dinner times.¹⁷ Furthermore, in the Nenne-meet group, the irregularity of daytime schedules, including social jetlag, was smaller than in the Nenne-not-meet group. These results are consistent with previous studies that children with higher social jetlag had more behavioral problems.^16,17 Therefore, it is important to avoid expanding the social jetlag for the mental and physical development of toddlers. In the Nenne-meet group, toddlers spent more time engaging in sadative activities before bedtime and in bed than in the Nenne-not-meet group (Table 5). The establishment of a bedtime routine is reportedly associated with an earlier bedtime, a shorter sleep-onset latency, fewer nocturnal awakenings, and a longer sleep duration.³⁹ The results of this study emphasize the importance of the sedative activities, such as reading picture books as a bedtime routine.

As reported in a previous study,²³ in both groups, if the toddler had more older siblings, they were prone to sleep problems. These results suggest that it is necessary to consider not only the daily schedules of toddlers and caregivers but also background factors, such as family composition, to develop appropriate sleep habits in Japanese toddlers. In some cases, we should give advice involving siblings.

Factors for good sleep in Japanese toddlers

Based on previous reports,^{11,15–17,22} the following 4 factors influenced each other.

Sleep pressure

Sleep pressure, the major determinant of smooth sleep onset, was influenced by the following 2 domains: “Hyperarousal” and “Irregularity of sleep.” This result was consistent with previous reports.^15–17,22

Hyperarousal

We found that “Hyperarousal” was affected by the habit of media viewing. This was consistent with the results of previous studies that showed that longer durations of TV viewing at night were associated with later bedtimes and wake-up times²² and media use in the evening has a negative impact on sleep.²⁰ In practice, it is very difficult to control media use since it is a convenient tool for childcare.⁴⁰ Caregivers have reported feeling guilty when using smartphones to calm their children.⁴⁰ Control of media use and education on media literacy are needed from early childhood.⁴¹ Therefore, the results of the present study indicated that practical guidance for media control is important for parents of toddlers.

Owens et al¹³ reported that TV viewing immediately before bedtime is associated with frequent nocturnal awakenings in participants aged 4−10 years. However, in this study, we could not demonstrate a direct association between nocturnal awakenings and “Hyperarousal.” In this study, we regarded more than 5 minutes of awakening as nocturnal awakening, and brief periods of fussiness were not counted. This may account for the absent positive correlation between hyperarousal behaviors and nocturnal awakening. On the other hand, Cheung et al⁴² reported that touchscreen use and TV exposure did not affect the frequency of nocturnal awakenings in children aged 6–36 months, similar to the results of this study. Therefore, sensitivity or excitability related to media use may vary according to age.

Irregularity of sleep

Furthermore, we found that “Irregularities of sleep” negatively affect “Sleep pressure.” This result reinforced that, in addition to reducing irregularities at bedtime, it is important to ensure regularity in the time of the toddler’s dining, baths, naps, and waking up since these factors greatly affect sleep rhythms and daytime behavior.^{2,3,5,6,16,18} Previous studies reported that bathing was a recommended habit¹⁵ or bedtime routine.^43,44 Japanese-style bathing, which involves soaking in hot water up to the shoulders in deep bathtubs for a long time, is different from Western-style bathing⁴⁵ and it takes a relatively longer time than taking shower. Japanese individuals take baths daily, usually before bedtime.⁴⁵ Even in this study, 96.0% of toddlers not only took a shower but also soaked in a bathtub. So, taking a bath is one of the biggest issues for pre-bedtime activity, as shown in Figure 1.

White/Caucasian children had more regular bedtime than Hispanic, Asian, and Black children.³⁶ White and non-Hispanic children had earlier and more regular bedtimes, had longer nocturnal sleep duration, and took fewer naps compared with most racial and ethnic minorities.⁴⁶ In Japan, maternal employment status affected the irregularity of the daily schedule⁴⁷ because the mothers’ return time fluctuates. We previously reported that the daily schedules of children were influenced by the lifestyle of the parents.⁹ The late bedtime and late wake-up time of toddlers on weekends were affected by their caregivers’ daily schedule in Japan. In individual guidance on sleep hygiene, we recommend that the caregivers maintain regularity of the daily schedules of not only their children but also the caregivers themselves. Our next task is development of knowledge about the good sleep habituation of toddlers into Japanese society.

There are many reports on the short nocturnal sleep duration of Asian children.^8,33 It is possible that biologically Asian children sleep fewer hours than White children. It was reported that the duration and structure of sleep have a substantial genetic component.^48,49 To clarify this, further investigation is needed.

Conditioning

This study showed that awakening after sleep onset was strongly associated with the latent variable “Conditioning.” In this study, we revealed that the most important factor for conditioning is breastfeeding. This was consistent with previous reports that showed that breastfeeding in response to nocturnal awakening is associated with increased sleep fragmentation²² and a higher number of nocturnal awakenings was predominantly associated with breastfeeding for going back to sleep,⁵⁰ even in Japanese toddlers.¹⁴ A previous study reported that the rate of breastfeeding 15-month-old toddlers was higher in Japan than in the United States and France (Japan, ∼20%; United States, ∼15%; and France, <5%).⁵¹ In this study, 19.2% of the children were still breastfed. Such elongated breastfeeding periods may lead to differences in sleep behaviors in both White-dominant countries and Japan. Toddlers’ crying at night is one of the most serious problems for mothers in any country.^31,32 Therefore, in Japan, bedtime routines other than breastfeeding should be encouraged, and effective methods for caregivers to put toddlers back to sleep should be proposed.

Limitations and future directions

The limitations of this study are as follows: in the present study, the sleep–wake time, including nocturnal awakening, was only reported subjectively by the caregivers without using objective methods such as actigraphy. We confirmed that the report by the caregiver was correct for the sleep-onset and wake-up time; however, this might not be true for nocturnal awakenings.²⁵ This could exert a significant influence on the analysis of the night awakenings. Second, since we elicited information on the toddlers’ daytime behavior, such as mood upon waking, appetite for breakfast, and mood during the day, we could not evaluate this information by standardized questionnaires. Third, we adjusted for social aspects, such as daycare attendance, etc; however, other factors that could have affected the participants’ sleep behavior were not evaluated by us. Therefore, additional work is needed to confirm this.

Finally, there is a possible selection bias toward caregivers who were familiar with the Internet. Although the household income level was similar to the Japanese average income, the academic level of the caregivers was higher in this study than the average level for the same generation. In addition, online surveys are considered to be less reliable than face-to-face surveys.

CONCLUSIONS

The results of this study indicated that the Nenne-criteria are useful for ascertaining the minimal criteria for intervention, ie, the total sleep duration alone, for toddlers’ adequate sleep in Japan. To intervene with the help of caregivers for improving the toddlers’ sleep, it is important not only to present the ideal nocturnal sleep duration but also to give the caregivers tips (eg, Table S8 ^{(476.2KB, pdf)} ) for early bedtimes by increasing the sleep pressure at an appropriate time by waking-up early, ending naps earlier, etc, and reducing irregularity of sleep and to reduce nocturnal awakening by preventing hyperarousal and conditioning.

DISCLOSURE STATEMENT

All authors approved the manuscript as submitted and agree to be accountable for all aspects of the work. Work for this study was performed at the Department of Child Development, United Graduate School of Child Development, Osaka University, Osaka, Japan. This work was supported in part by research grants from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (16H03273 to M. Taniike, 17K17855 to A.Y., 18K15669 to E.M., and 22K15898 to E.M.); the Center of Innovation Program from the Japan Science and Technology Agency (JST), Japan (Center of Innovation [COI]: grant number JPMJCE1310 to M. Taniike); the National Institute of Information and Communications Technology (NICT), Japan (20008 to M. Tachibana). The authors report no conflicts of interest.

ABBREVIATIONS

NSF

National Sleep Foundation

SD

standard deviations

SEM

structural equation modeling

TV

television