Most parents learn pretty quickly that an overtired child is an unruly child. Now, a new study finds that children's behaviour problems might be linked not just to how much sleep they get, but whether they have a regular bedtime.

New research published in the journal Pediatrics finds that kids who don't have a regular bedtime are more likely to have behaviour problems than kids with a set time for lights-out.

And the longer a child goes without a regular bedtime, the worse his or her behaviour problems become over time, the study found.

The authors of the study say that regular bedtime routines appear to be key to a child's brain development. They say irregular bed times can disrupt natural body rhythms, leading to sleep deprivation, which can then undermine the brain's ability to mature and to regulate certain behaviours.

To a child, a disrupted sleep schedule can cause a kind of "jet lag," says the study's lead author, Yvonne Kelly, a professor of life course epidemiology at University College London.

"Not having fixed bedtimes, accompanied by a constant sense of flux, induces a state of body and mind akin to jet lag and this matters for healthy development and daily functioning," she says in a statement.

Sleep disruptions, especially if they occur at key times in a child's development, could have important lifelong impacts on health, Kelly says.

The good news from the study, though, is that these effects can be reversed if a good bedtime routine is established.

For the study, Kelly and her colleagues reviewed data on more than 10,000 7-year-olds who were enrolled in the U.K. Millennium Cohort Study.

Details on the children's bedtimes were collected when they were 3, 5 and 7 years old. Researchers also asked teachers and mothers to rate the children's behaviours at the same time that they collected data on bedtimes.

They asked the teachers and mothers to evaluate the children's behaviour based on 25 questions that asked about conduct problems, problems with peers, emotional difficulties and hyperactivity. The researchers did not collect data on how much sleep the children received overall.

The researchers found that the kids with irregular bedtimes had more behavioural problems than kids with regular bedtimes, according to both their teachers and their mothers.

On average, a child who had an irregular bedtime at age 3 or 5 increased his or her score on the behavioural difficulties scale by about a half-point at the next time-point. If that child had an irregular bedtime at two time-points during the study, the score increased by about 1 point. If the child had an irregular bedtime at all three time-points during the study, the score increased by just over two points.

"What we've shown is that these effects build up incrementally over childhood, so that children who always had irregular bedtimes were worse off than those children who did have a regular bedtime at one or two of the ages when they were surveyed," Kelly said.

But children who began to have regular bedtimes also showed improvements in their behaviour over the course of the study.

The study found that it was not unusual for children to have irregular bedtimes at the age of three; around one in five children went to bed at varying times at that age. By age seven, however, more than half the children went to bed regularly between 7:30 and 8:30 p.m.