Researchers have found that inconsistent bedtimes are linked to children's cognitive development. The authors warned that there could be “knock-on” health effects throughout life.
They wanted to assess whether the time a child went to bed, and the consistency of bed-times, had any impact on their intellectual performance, measured by testing their reading, maths and spatial awareness.
The study, published in the Journal of Epidemiology and Community Health, examined more than 10,000 children who are taking part in the Millennium Cohort Study - a long-term study of children born in the UK between September 2000 and January 2002.
The research was drawn from regular surveys and home visits made when the children were three, five and seven to find out about family routines, including bedtimes.
When children were three, almost one in five had irregular bedtimes but the figure reduced to less than one in 10 when the children were older.
By the age of seven, more than half the children went to bed regularly between 7.30pm and 8.30pm.
Children without regular bedtimes and who were put to bed after 9pm tended to come from more socially disadvantaged backgrounds, the researchers said.
They found that seven-year-old girls who had irregular bedtimes had lower scores on all three aspects of intellect assessed compared to children who went to bed at a regular time. But the effect was not found in boys.
Non-regular bedtimes at age three were associated with lower reading, maths and spatial awareness scores in both boys and girls.
Girls who had never had regular bedtimes at ages three, five and seven had significantly lower reading, maths and spatial awareness scores than girls who had had consistent bedtimes. For boys this was the case for those having non-regular bedtimes at any two ages.
But they also found that irregular bedtimes by the age of five were not associated with poorer brain power in girls or boys at the age of seven.
“Sleep has a crucial and complex role in the maintenance of health and optimal function,” the authors wrote.
“Inconsistent bedtime schedules might impact on markers of cognitive development in two ways, via disruptions to circadian rhythms and/or sleep deprivation and associated effects on brain plasticity.”
They concluded: “We show that a range of social and family environmental factors are related to bedtimes during early childhood.
Consultant paediatrician Dr Robert Scott-Jupp, from the Royal College of Paediatrics and Child Health, said: “At first glance, this research might seem to suggest that less sleep makes children less intelligent, however, it is clearly more complicated than that.
“There are a variety of reasons as to why children who aren't as clever might sleep less, such as family income, parents' level of education and mental health with this research confirming a strong link between social disadvantage and late bedtimes.
“Researchers did go to great lengths to try to allow for this statistically yet still found the relationship between lack of sleep and development was apparent - and more so for girls.
“While it's likely that social and biological brain development factors are inter-related in a complex way, in my opinion, for school children to perform their best, they should all, whatever their background, get a good night's sleep.”
The consistent nature of bedtimes during early childhood is related to cognitive performance. Given the importance of early child development, there may be knock on effects for health throughout life.