What happens in the early years of a person’s life has a profound effect on how they fare later on. Thousands of research papers – many of them using the rich data in the British Birth Cohort studies – have shown that children who get a poor start in life are much more likely to experience difficulties as adults; whether that’s to do with poor health, or their ability to enjoy work and family life.

Ensuring that children get enough sleep is one of a number of ways to get them off to the best possible start in life. The National Sleep Foundation recommends that toddlers should get roughly 11 to 14 hours of sleep every day. For children aged three to five years, the recommendation is ten to 13 hours, or nine to 11 hours for children once they’re at primary school.

But the latest research carried out by our team at UCL’s International Centre for Lifecourse Studies, shows that it’s not just the amount of sleep a child gets which matters. After digging into the data from the Millennium Cohort Study (MCS) – which has followed the lives of some 20,000 children since the turn of the century – we found that having a regular bedtime also affects how they get on at home and at school, throughout the first decade of their lives.

Image: Sleep Foundation

The ‘jet lag’ effect

To begin with, we looked at the relationship between regular and irregular bedtimes, and how the children got on in a range of cognitive tests. 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, while those who answered “sometimes” or “never” were put in the irregular bedtime group.

The results were striking. Children with irregular bedtimes had lower scores on maths, reading and spatial awareness tests. In fact, the time that children went to bed had little or no effect on their basic number skills, or their ability to work with shapes. But having no set bedtime was linked to lower scores, especially for three-year-olds. The greatest dip in test results was seen in girls who had no set bedtime throughout their early life.

At the heart of this phenomenon is the circadian rhythm – the internal body clock, which tells you when it’s time to sleep and wake up.

If I travel from London to New York, I’m likely to be slightly ragged when I arrive, because jet lag is going to affect my cognitive abilities, appetite and emotions. If I bring one of my children with me, and I want them to do well at a maths test having just jumped across time zones, they will struggle even more than I will. If we think of the body is an instrument, then a child’s body is more prone to getting out of tune.

The same thing happens when children go to bed at 8pm one night, 10pm the next and 7pm another . Scientists sometimes call this the “social jet lag effect”. Without ever getting on a plane, a child’s bodily systems get shuffled through different time zones, and their circadian rhythms and hormonal systems take a hit as a result.

Best behaviour

As well as enhancing a child’s intellectual development, we found that regular bedtimes can also improve their behaviour.

At age seven, according to parents and teachers, children in the MCS who had irregular bedtimes were considerably more likely to have behavioural problems than their peers who had a regular bedtime. The more frequently a child had been able to go to bed at different times each night, the worse his or her behavioural problems were. In other words, the effects appeared to accumulate throughout childhood.

But we did find an important piece of good news, too: those negative effects on behaviour appeared to be reversible. Children who switched to having a regular bedtime showed improvements in their behaviour. This shows that it’s never too late to help children back onto a positive path, and a small change could make a big difference to how well they get on.

But of course, the reverse was also true: the behaviour of children who switched from a regular to an irregular bedtime got worse.

A weighty problem

In a follow-up study, which looked at the impact of routines (including bedtimes) on obesity, we reported that children with irregular bedtimes were more likely to be overweight, and have lower self-esteem and satisfaction with their bodies.

In fact, of all the routines we studied, an inconsistent bedtime was most strongly associated with the risk of obesity. This supports other recent findings, which show that young children who skipped breakfast and went to bed at irregular times were more likely to be obese at age 11. Even children who “usually” had a regular bedtime were 20% more likely to be obese than those who “always” went to bed at around the same time.