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Irregular bedtimes may limit children's brainpower, a new study suggests.

British researchers looked at whether bedtimes in early childhood were related to brainpower in more than 11,000 seven-year-olds.

Researchers wanted to see whether the time a child went to bed, and the consistency of bedtimes affected intellectual performance, measured by validated test scores for reading, mathematics and spatial awareness.

Researchers also wanted to know if the effects were cumulative and/or whether any particular periods during early childhood were more critical than other.

The findings revealed that irregular bedtimes were most common at the age of three, when around one in five children went to bed at varying times.  Researchers noted that by age seven, more than half the children went to bed regularly between 7:30pm and 8:30 pm.

The study showed that children whose bedtimes were irregular or who went to bed after 9:00 pm came from more socially disadvantaged backgrounds.

The study found that at the age of seven, girls who had irregular bedtimes had lower scores for reading, mathematics and spatial awareness, after taking account of other potentially influential factors, than children with regular bedtimes. However, researchers noted that this was not the case in seven-year-old boys.

The study found that irregular bedtimes by the age of 5 were not associated with poorer brainpower in girls or boys at the age of 7.

However, irregular bedtimes at 3 years of age were associated with lower scores in reading, mathematics, and spatial awareness in both boys and girls.  Researchers said the findings suggest that around the age of three could be a sensitive period for cognitive development.

The findings also suggest that the impact of irregular bedtimes is cumulative.

Researchers found that girls who never had regular bedtimes at ages 3, 5 and 7 had significantly lower reading, mathematical and spatial awareness scores than girls who had consistent bedtimes. Researchers found that the impact was the same in boys, but for any two of the three time points.

Researchers say irregular bedtimes could mess up the body's natural rhythms and cause sleep deprivation, therefore undermining the plasticity of the brain and the ability to obtain and retain information.

"Sleep is the price we pay for plasticity on the prior day and the investment needed to allow learning fresh the next day," researchers wrote.

"Early child development has profound influences on health and wellbeing across the life course. Therefore, reduced or disrupted sleep, especially if it occurs at key times in development, could have important impacts on health throughout life," they concluded.

The findings are published in the Journal of Epidemiology and Community Health.