Irregular bedtimes may disrupt healthy brain development in young children, according to a study of intelligence and sleeping habits.
Going to bed at a different time each night affected girls more than boys, but both fared worse on mental tasks than children who had a set bedtime, researchers found.
The effect was most striking in three year olds, where boys and girls scored lower on reading, maths and spatial skills tests than children of the same age who kept to a more rigid schedule.
Scientists at University College London said the lack of routine might impair early development by disrupting the body clock, or through sleep deprivation, which affects the brain's ability to remember and learn new information.
"Age three seems to be where you see the largest effect and that is a concern," said Amanda Sacker, professor of lifecourse studies at UCL.
"If a child is having irregular bedtimes at a young age, they're not synthesising all the information around them at that age, and they've got a harder job to do when they are older. It sets them off on a more difficult path," she added.
While the differences in test scores were modest – only a few points in many cases – irregular bedtimes throughout childhood appeared to have a cumulative effect, leading to greater problems later on.
Sacker and her colleagues drew on information in the UK Millennium Cohort Study (MCS), a long-term record of UK children who are now in or approaching their early teenage years. They looked at how the children's performance fared on tests at various ages, and whether bedtime had any impact.
Parents who took part in the MCS were asked whether their children went to bed at a regular time on weekdays. Those who answered "always" or "usually" were put in the regular bedtime group in Sacker's study, while those who answered "sometimes" or "never" were put in the irregular bedtime group.
The hour that children went to bed had little or no effect on their performance on different tests, including basic number skills, reading out word cards, and constructing designs from flat or solid shapes. But having no set bedtime often led to lower scores. The greatest dip in test results was seen in girls who had no set bedtime throughout early life, at three, five and seven years old. The study found the same for boys who had irregular bedtimes at any two of these ages.
Writing in the Journal of Epidemiology and Community Health, the authors suggest that irregular bedtimes affect the brain's "plasticity", or ability to store and learn new information. "Sleep is the price we pay for plasticity on the prior day and the investment needed to allow fresh learning the next day," the authors write. "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.,"