The brain power of children may be adversely affected if they go to bed at different times every night, a new study indicates.

UK scientists decided to investigate whether the time a child went to bed had any effect on their intellectual performance. They looked at over 11,000 seven-year-olds, all of whom were taking part in an ongoing study which had monitored their daily routines, including bedtimes, at the ages of three, five and seven.

The study found that children were most likely to have an irregular bedtime at the age of three. In fact, one in five three-year-olds regularly went to bed at different times. By the age of seven, over half had a regular bedtime of between 7.30 and 8.30pm.

The study also noted that children who had more irregular bedtimes or who went to bed after 9pm at night tended to come from more socially disadvantaged homes.

The children's intellect was tested using standard tests, which looked at their abilities in maths, reading and spatial awareness.

The scientists from University College London found that by the time they were seven years of age, girls who had irregular bedtimes scored lower in all three areas - maths, reading and spatial awareness - compared with children who had regular bedtimes.

This was not the case with seven-year-old boys. However, both boys and girls who had irregular bedtimes at the age of three were more likely to score poorly in the intellect tests. This suggests that the age of three is an important period for cognitive development, the scientists suggested.

Meanwhile the study also found that the effects of irregular bedtimes could be cumulative. Girls who had never had a regular bedtime at the ages of three, five and seven scored significantly lower in the intellect tests compared to girls who had regular bedtimes.

In boys, the impact was the same for two of the three ages tested.

The scientists believe that irregular bedtimes may adversely affect children's brain power because they disturb natural body rhythms and cause sleep deprivation. This may affect the brain's ability to process information.

"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," the team said.

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