That's according to a new study, published in BMJ.
And most of that added weight is stored fat, not muscle.
Sleep appeared to influence a child’s body weight more than both diet and physical activity. Only maternal weight mattered more, in the researchers' final analysis.
“The magnitude of the sleep effect was bigger than I expected,” says study researcher Barry Taylor, MBChB, head of the department of women and children’s health in the Dunedin School of Medicine at the University of Otago in New Zealand.
Experts who were not involved in the study say the findings are an important contribution, and if anything, probably underestimate the effect of sleep on weight.
“I think it’s consistent with what’s been seen in some of the other research that’s been focused on this age group,” says Michelle Garrison, PhD, a research scientist at Seattle Children’s Research Institute. “Poor sleep during the preschool ages really does seem to be associated with subsequent increased BMI scores.”
The study didn’t account for periods of time that children might spend awake in the middle of the night in their beds, Garrison points out.
“Night wakings can be pretty common in kids this age," she says. For example, she says a child that has a "sleep duration" of 11 hours might really have only slept for 9.5 of those hours, after taking the child's night wakings into account.
But "that difference doesn’t discount their findings," Garrison tells WebMD. "If anything, I think if they had looked at true total sleep, the findings would have been even stronger."
Tracking Childhood Sleep and Body Weight
For the study, researchers followed 244 children from ages 3 through 7, regularly checking their sleep time, physical activity, diet, body mass, and fat distribution.
Unlike previous research, which has typically relied on parents to report how long children are sleeping, this study also used a more direct measure: movement sensors that were worn by the children around their waists.
Researchers found that children slept an average of about 11 hours daily, with nearly all children in the study sleeping somewhere between 9.5 hours and 12.5 hours each day, including naps.
Even after accounting for diet, physical activity, and a host of other factors known to influence weight, each additional hour of daily sleep children logged from age 3 to age 5 was associated with about a half-point drop in the kids' body mass index (BMI) by age 7.
In a child of average height, that amounts to 1.5 pounds.
Children at the low end of the sleep scale had more body fat than children who got more sleep. There was no difference in muscle between shorter and longer sleepers.
Advice to Parents
Though this study shows a link between children's sleep and body weight, it doesn't prove that getting less sleep causes weight gain. And the researchers stopped short of offering parents advice about how long kids should be sleeping or what to do if they are worried that their children aren’t sleeping enough.
According to the National Sleep Foundation, children need between 11 and 13 hours of sleep for kids ages 3-5.
If children don’t seem to be hitting those targets, Garrison says parents should look at lifestyle factors that might be hampering a regular sleep routine.
“For kids this age, a lot of things can affect sleep quality, like TV use and physical activity,” she says.