"If the child prefers to go to sleep a little bit later, but it's done regularly, that's still OK for them, according to the evidence," said Amanda Sacker, professor in the Department of Epidemiology and Public Health at University College London.
Researchers looked at information about bedtimes and standardized test scores for more than 11,000 children who were part of the UK Millennium Cohort Study, a nationally representative study of children in the United Kingdom.
The Millennium Cohort Study followed children when they were aged 3, 5 and 7, and included regular surveys and home visits. Researchers asked parents about family routines such as bedtimes.
Children also took standardized tests in math, reading and spatial abilities when they were 7 years old.
Researchers controlled for socioeconomic status in addition to other factors such as discipline strategies, reading to children and breakfast routines.
The study found that, in general, consistent bedtimes were linked to better performance across all subject areas. This was especially true for 7-year-old girls, regardless of socioeconomic background - they tended to do worse on all three intellect measurements if they had irregular bedtimes. Boys in this age group did not show the effect.
In both girls and boys, non-regular bedtimes at age 3 were linked with lower test scores, but not at age 5.
Bedtimes that had never been consistent for girls at ages 3, 5, and 7 were associated with lower scores than regular bedtimes. For any two of these ages, boys also tended to do worse on the tests if they didn't go to sleep at a routine time.
These results "showed that it wasn't going to bed late that was affecting child's development, it was the irregular bedtimes that were linked to poorer developmental scores," Sacker said.
Image: Boy at bedtime with clock, via Shutterstock