Kids without a regular bedtime tend to have more behavior problems at home and at school, a new study suggests.
Researchers found that when children started going to sleep at a more consistent time, their behavior improved as well.
"If you are constantly changing the amounts of sleep you get or the different times you go to bed, it's likely to mess up your body clock," said Yvonne Kelly, who led the study.
"That has all sorts of impacts on how your body is able to work the following day," Kelly, from University College London, said.
She and her colleagues analyzed data on more than 10,000 children. They were part of a long-term study of babies born in the UK in 2000 to 2002 that regularly surveyed parents about sleep and behavioral problems.
Children with attention-deficit/hyperactivity disorder or an autism spectrum disorder were not included in the study.
When kids were three years old, close to 20 percent of parents said their child sometimes or never went to bed at a consistent time. That fell to 9 percent among five-year-olds and 8 percent for seven-year-olds.
Children without a regular bedtime tended to score worse on a measure of behavior problems such as acting unhappy, getting into fights and being inconsiderate. The assessment is scored from 0 to 40, with higher scores indicating more problems.
When children were seven years old, for example, those without a regular bedtime scored an 8.5, on average, based on their mothers' reports. That compared to scores between 6.3 and 6.9 for kids who had a consistent bedtime before 9 p.m.
The researchers said one to two points represents a small or moderate difference, but is "meaningful."
Teachers of seven-year-olds were asked to report on their behavior as well. They also gave worse scores to children who didn't have a regular bedtime.
Kids whose parents said they had non-regular bedtimes on every survey growing up had the most behavioral issues, Kelly's team reported in Pediatrics.
But when children went from having a non-regular bedtime to a regular bedtime on the following survey, their behavior scores improved.
That is encouraging, Kelly said, because it shows parents can make changes to affect their child's behavior.
Although the researchers accounted for other parent characteristics and family habits, it's still possible kids' behavior problems weren't directly caused by irregular bedtimes.
"It's very difficult to know whether or not from a study like this, is it literally the not having a regular bedtime schedule that was contributing to the difficulties or is it representative of a bigger picture?" said Jodi Mindell.
A pediatric sleep specialist from St. Joseph's University in Philadelphia, she was not involved in the new research.
Still, Mindell said, getting kids to bed early and at a consistent time is "really important."
Kelly recommended families avoid television and other screens before bed and get into a routine that includes a bedtime story and other calm activities.
"All of these things which are kind of just basic common sense are going to help," she said.
Having a bedtime routine that is quality family time also makes it something children will look forward to, Mindell said.
"I think that parents need to make sleep a priority, and they need to realize that it has huge ramifications not only that evening, but the next day, the next week, the next year," she said.
However, parents don't have to drive themselves crazy, Kelly said. "The odd late night is not going to cause harm either," she said.