A regular bedtime may be important for the cognitive development of young children, researchers found.
Girls -- but not boys -- who didn't have a regular bedtime at age 7 had slightly but significantly lower scores for reading, math, and spatial abilities (by 15% to 26% of a standard deviation), according to Yvonne Kelly, PhD, of University College London, and colleagues.
The effect appeared to accumulate, because a failure to go to bed at a regular time at multiple time points in the first 7 years of life was associated with lower cognitive scores for both boys and girls, the researchers reported online in the Journal of Epidemiology & Community Health.
Not going to bed on a schedule "could have important ramifications, as when sleep is restricted or disrupted symptoms that reflect a reduced capacity for plastic change [in the brain] and/or disrupted circadian rhythms follow, including cognitive impairment and lack of concentration," they wrote.
"Early child development has profound influences on health and well-being across the life course," they noted. "Therefore, reduced or disrupted sleep, especially if it occurs at key times in development, could have important impacts on health throughout life."
The study included 11,178 7-year-olds living in the U.K. who were tracked from infancy as part of the longitudinal Millennium Cohort Study. Their parents were asked about bedtimes when the children were ages 3, 5, and 7. Cognitive assessments were performed at age 7.
For the current analysis, having a regular bedtime was defined as the parent saying the child always or usually went to bed at the same time each night (versus sometimes or never).
The percentage of children who did not have a regular bedtime declined with age from 19.5% at age 3 to 9.1% at age 5 to 8.2% at age 7.
Looking just at age 7, not having a regular bedtime was associated with significantly lower scores for reading (beta -0.22), math (beta -0.26), and spatial abilities (beta -0.15) among girls in a fully adjusted model. There were no significant relationships among boys.
Not having a regular bedtime at age 3, however, was associated with lower cognitive scores at age 7 for both girls and boys: reading (beta -0.10 for girls and -0.20 for boys), math (beta -0.16 and -0.11), and spatial abilities (beta -0.13 and -0.16).
An irregular bedtime at age 5 was associated only with lower scores for reading among girls (beta -0.15) and for math among boys (beta -0.14).
Not going to bed at the same time each night at multiple ages was associated with even greater effects on cognition.
Among girls, those who did not have a regular bedtime at ages 3, 5, and 7 had significantly lower scores for reading (beta -0.36), math (beta -0.51), and spatial abilities (beta -0.40) at age 7.
And among boys, those who did not have a regular bedtime at any two of those ages had significantly lower scores for reading (beta -0.28), math (beta -0.22), and spatial abilities (beta -0.26) at age 7.
"It might be that inconsistent bedtimes are a reflection of chaotic family settings and it is this, rather than disrupted sleep that impacts on cognitive performance in children," the authors wrote. "However, we found that inconsistent bedtimes were linked to markers of cognitive performance independent of multiple markers of stressful family environments."
Those markers included parental employment, parental views of the amount of time spent with the child, attendance at a breakfast club or after-school club, any other childcare, whether the child was read to or told stories, TV time rules, whether there was more than one child in the bedroom, bed wetting, and the presence of a TV in the bedroom.
"Thus," the researchers wrote, "our results suggest that having a regular bedtime is important alongside other aspects of family circumstances."
They acknowledged some limitations of the study, including the possibility of recall bias affecting the data on bedtimes, the lack of information on bedtimes on the weekend, the inability to reliably examine the possible effects of insufficient sleep, and the lack of direct data on sleep quantity and quality.
The Millennium Cohort Study is funded by grants from the Economic and Social Research Council.
The authors reported that they had no conflicts of interest.
- Reviewed by Zalman S. Agus, MD Emeritus Professor, Perelman School of Medicine at the University of Pennsylvania and Dorothy Caputo, MA, BSN, RN, Nurse Planner
Journal of Epidemiology & Community HealthSource Reference: Kelly Y, et al "Time for bed: associations with cognitive performance in 7-year-old children: a longitudinal population-based study" J Epidemiol Community Health 2013; DOI: 10.1136/jech-2012-202024.