This study focused on the association between mental health problems and sleep habits/problems in children aged 3 to 4 years. Based on our results, among 42-month children, the prominent sleep problems—anxiety before going to sleep, abnormality in the circadian rhythm, and sleepiness during classes outside of naptimes—were observed more frequently in those with poor mental health than in control participants. Moreover, their mental health status was significantly associated with sleepiness and snoring.
There were no significant differences in sleep duration, bedtime, wake time, or nap condition between the poor mental health group and the control group. A systematic review of the relationship between sleep duration and health indicators in children and youths aged 5–17 years showed that shorter sleep duration was associated with adverse physical and mental health, such as excess adiposity, poorer emotional regulation and academic achievement, and a lower quality of life [25]. In terms of the early years (0–4 years), shorter sleep duration is generally associated with higher adiposity, poorer emotional regulation, impaired growth, more screen time, and a higher risk of injuries, although the evidence is not sufficient and is mixed for cognitive development and physical activity [26]. Our study did not reveal a positive relationship between sleep duration and mental health, as scored using the SDQ, probably because of the age considered. No difference was found in sleep duration according to mental health status, as the children in both groups slept for more than 10 hours on average, which is within the range of the National Sleep Foundation’s recommendation that preschoolers (3–5 years) sleep for 10–13 h/day [6, 7].
In our study, there was also no difference in the pre-sleep domain, except for anxiety before going to sleep. Baum et al. reported that adolescents aged 14–17 years rated themselves as significantly more anxious, angry, confused, and fatigued, and less vigorous, when they had sleep restriction (6.5 hours in bed per night for five nights) compared with when they had healthy sleep (10 hours in bed per night for five nights) [27]. Another randomized trial showed that even modest differences in sleep duration over just a few nights can have significant consequences for daytime functioning, including emotion regulation, short-term memory, working memory, and aspects of attention in children aged 8–12 years [28]. The relationship between sleep problems and mood has been well established not only in adults but also in younger populations; however, evidence in preschool children is lacking. In addition, anxiety in children is difficult to measure even by their own parents, and might be fear or a manifestation of bedtime resistance.
In terms of the daytime domain, abnormality in the circadian rhythm and sleepiness negatively affected mental health in children aged 3 to 4 years. Sleep/wake patterns are influenced by both environmental and genetic factors that vary across different populations and cultures [29, 30, 31, 32]. Adolescent sleep-wake patterns have been associated with several factors, such as pubertal development [33, 34], decreased parental monitoring of bedtime [35], increased demands by the school schedule [32, 36], and changes in the circadian rhythm [37]. Moreover, children in families with lower socioeconomic status were found to exhibit a later rise time, longer time in bed, more nocturnal awakening, and more night-to-night variability in bedtime and time spent asleep [38]. However, in preschool children, there is insufficient data on abnormalities in the circadian rhythm, except regarding neurodevelopmental disorders, such as autism spectrum disorder [39, 40, 41] and attention-deficit/hyperactivity disorder [42].
The main finding of our study was that sleepiness and snoring were strongly associated with mental health status in children aged 3 to 4 years. Sleepiness is a very common complaint among schoolchildren, and data on its actual prevalence differ from study to study. The varying prevalence of daytime sleepiness depends on the questions used, sample sizes, study area, year the survey was conducted, age, and ethnicity. Moreover, each study can capture only limited aspects of sleepiness, and this must be considered when interpreting the results [43]. Sleepiness is also related to other sleep disturbances, such as narcolepsy [44], sleep-disordered breathing [45], and restless legs syndrome [46].
Snoring is the major clinical symptom of obstructive sleep apnea (OSA), which is a condition that is characterized by recurrent episodes of gas exchange abnormalities and repeated arousals that affect between 1% and 3% of 2- to 8-year-old children [47, 48, 49]. Children who snore but do not fulfill the criteria for OSA are considered to have primary snoring [50]. Daytime sleepiness, behavioral hyperactivity, learning problems, and restless sleep are all significantly more common in habitual snorers [47, 51, 52]. Poor mental health, daytime sleepiness, and snoring might be related to each other. Sleep disturbances, especially OSA, should be taken into account as a cause of poor mental health in early childhood. In addition, not only OSA but also primary snoring should be considered as a cause of poor mental health in children aged 3–4 years.
Our findings have several clinical implications. It is difficult for parents, especially first-time parents, to notice abnormalities in their child’s circadian rhythm. Parents tend to think that their children’s circadian rhythms are natural and take them for granted; therefore, parents do not consult anyone, and the children and their parents gradually become exhausted. Moreover, an abnormality in the circadian rhythm affects daytime functioning and leads to poor mental health. Once parents can understand that abnormality in circadian rhythm is not because of the way they have raised their children, they will easily accept the sleep disturbances or poor sleep hygiene. Interventions for better sleep hygiene could then be accepted by children and their parents, which would consequently constitute an early intervention for the children’s development. We hope that our findings hope to promote not only good sleep hygiene but also early intervention for mental health and development. However, more research is needed to address the longitudinal changes in the developmental trajectories and sleep habits/problems.
This study had several limitations. First, the cutoff points of the SDQ scores for 2–4-year-old children were defined in the UK, and Japanese children might have different cutoff points. Indeed, the percentages of the very high and high groups were 1.2% and 3.4%, respectively, which were smaller than 4% and 4% in the UK, respectively [23]. Second, most children attended nursery schools or kindergartens, and the parents did not know how their children spent their day and could not evaluate children’s activity during the day precisely. Third, sleepiness is difficult to assess, especially in children, as they cannot express their subjective sleepiness themselves and tend to be more hyperactive than usual when they are sleepier. Preschool children are notoriously difficult to observe in laboratory settings when using polysomnography for nighttime sleep and the multiple sleep latency test for daytime sleepiness. Fourth, we did not investigate the presence or absence of neurodevelopmental disorders. Therefore, future work might consider using more objective approaches, such as actigraphy or portable sleep study equipment, for sleep evaluation. Moreover, the association between sleep-related problems and neurodevelopmental disorders should be addressed.