Infants and toddlers who consistently sleep fewer hours than age-appropriate recommendations face significantly higher odds of developmental delays by age 3, according to a study published July 8 in Frontiers in Public Health that draws on Japan’s largest nationwide birth cohort.
The finding adds the weight of population-scale evidence to a question that has long concerned pediatricians and sleep researchers: whether short sleep in early life is merely a marker of something else going on, or an independent risk factor for how a child’s brain develops.
What they found
Researchers led by Toshio Masumoto at Tottori University analyzed data from the Japan Environment and Children’s Study, or JECS, a government-funded prospective birth cohort that enrolled more than 100,000 pregnant women across 15 regional centers between 2011 and 2014. The team focused on a subset of roughly 63,000 to 64,000 children whose sleep duration was reported by caregivers at 6 months, 1 year, 1.5 years, and 3 years of age.
Developmental outcomes were measured at age 3 using the Ages and Stages Questionnaires (ASQ-3), a well-validated screening tool that assesses five domains: communication, gross motor, fine motor, problem solving, and personal-social skills.
After dividing children into age-specific sleep duration categories based on consensus recommendations, the pattern was clear. Children who slept fewer than 10 hours per night at any of the four measurement points showed higher odds of scoring below the developmental screening threshold at age 3.
A closer look at the numbers reveals a consistent signal. Short sleep at 6 months carried an odds ratio of 1.11 (p<0.001) for developmental delay. At 1 year the odds ratio was 1.09 (p=0.01). By 3 years the gap had widened: an odds ratio of 1.15 (p<0.001). The association held after controlling for a wide range of potential confounders including maternal age, education level, household income, medical history, smoking during pregnancy, the child’s birth weight, and sex.
The analysis also looked at sleep location. Infants who shared a room with parents or slept in a separate room both tended to sleep longer than those who co-slept in the same bed, though the study notes that sleep location’s relationship with developmental outcomes was complex and likely mediated by other factors.
Why it matters
The JECS cohort is not a small convenience sample. Its size and nationwide scope mean the findings reflect real-world patterns across Japan’s socioeconomic and geographic diversity, not just one clinic’s patient population. That scale matters because small effects in early childhood can translate into meaningful population-level differences.
The observed odds ratios of 1.09 to 1.15 are modest in magnitude, but sleep is a modifiable behavior. Unlike genetic predisposition or socioeconomic background, sleep habits are something families and clinicians can address directly. Pediatric sleep interventions are well studied, relatively low cost, and do not require specialized equipment or pharmaceuticals.
The study also adds to a growing body of evidence linking early sleep patterns to later cognitive and behavioral outcomes. Previous work has tied short sleep in infancy to problems with attention, executive function, and emotional regulation in preschool and school-age children. The JECS analysis extends that pattern to a large Asian cohort, complementing research from Europe and North America that has been dominated by smaller samples.
Limits
The study has important caveats. Sleep duration was reported by caregivers, not measured objectively with actigraphy or polysomnography, and parents may overestimate or underestimate how long their child actually sleeps. The ASQ-3 is a screening tool, not a diagnostic assessment, which means it flags possible delays rather than confirming them. Some children identified as at risk in this study would not go on to receive a formal diagnosis.
The authors also acknowledge that while they controlled for many confounders, observational studies cannot prove causation. It remains possible that undiagnosed developmental issues themselves disrupt sleep, rather than the reverse. Reverse causation and residual confounding by unmeasured variables such as parental mental health, screen time, or daycare environment cannot be ruled out.
Additionally, the recommended sleep duration categories used in the study (e.g., 12 to 13 hours for 3-year-olds) are guidelines, not hard thresholds. Individual children vary in their sleep needs, and a child who sleeps somewhat less than the reference range is not necessarily at risk.
Bottom line
Shorter sleep during infancy and toddlerhood is associated with a statistically significant increase in the odds of developmental delay at age 3, even after adjusting for maternal and child factors. The effect is modest but consistent across multiple measurement points. Given that sleep is a modifiable behavior, the findings support routine counseling on age-appropriate sleep durations during well-child visits.
Parents concerned about their child’s sleep habits should discuss them with a pediatrician rather than attempting sleep interventions on their own. The study does not suggest that parents should wake sleeping children to meet a target, nor that occasional short nights are a cause for alarm. But the pattern across nearly 64,000 children is one that pediatricians and public health officials would be wise to take seriously.
Source
Masumoto T, Amano H, Otani S, Kurozawa Y, Morita A. “Shorter sleep duration in infancy and toddlerhood and the risk of developmental delay at age 3 years: the Japan Environment and Children’s Study.” Frontiers in Public Health. 2026 Jul 8;14:1736659. doi: 10.3389/fpubh.2026.1736659.