Little ones who stay up late may have a higher risk of becoming overweight by the time they are school-age, a new study suggests.
Researchers found that young children who routinely got to sleep after 9 p.m. tended to gain more body fat between the ages of 2 and 6. Compared with kids who had earlier bedtimes, they had bigger increases in both waist size and body mass index (BMI) -- an estimate of body fat based on height and weight.
The findings do not prove that later bedtimes cause excess weight gain, said Dr. Nicole Glaser, who wrote a commentary accompanying the study, which was published online Feb. 18 in Pediatrics.
But the report adds to evidence linking sleep habits to kids' weight, according to Glaser, a pediatric endocrinologist at the University of California, Davis.
Specifically, studies have found higher rates of obesity among kids who either get too little sleep or have trouble falling or staying asleep.
"At this point, I think it's clear that there is a relationship between [sleep quality and obesity risk]," Glaser said. "The big question is whether the relationship is a causal one."
Dr. Claude Marcus, senior researcher on the study, agreed. "The causality is difficult to establish," he said.
Kids' sleep habits do not exist in a vacuum, and it's possible that other factors cause both late bedtimes and greater weight gain, said Marcus, a professor of pediatrics at the Karolinska Institute in Stockholm, Sweden.
For example, it's possible that kids who stay up late are eating more at night, or that their parents put fewer limits on them in general, he said.
Marcus also pointed to stress, which could feed both poor sleep and overeating.
Then there is what's happening in the brain. Glaser said research shows that several brain areas that regulate the sleep-wake cycle also help govern appetite -- and some of the same hormones and brain chemicals are involved.
It's "certainly possible," she said, that poorer sleep alters young children's metabolism -- or behavior, like physical activity during the day.
But, Glaser added, "it's equally possible that the association between sleep patterns and obesity simply reflects the fact that similar brain centers are involved in modulating both."
The findings are based on 107 young children who were part of an obesity prevention project. Sixty-four had overweight or obese parents, so they were considered at high risk for excessive weight gain.
Between the ages of 2 and 6, kids' sleep habits were recorded for one week each year, with the help of a wrist device that monitors activity.
On average, the study found, children who routinely went to bed after 9 p.m. showed somewhat greater gains in BMI and waist size over the years. The link was independent of total time asleep, and it remained even after the researchers accounted for factors like kids' exercise habits and "screen time," and parents' education levels.
The connection was stronger among children whose parents were obese. Their waist size grew by an average of 3.5 centimeters (1.4 inches) more, compared to kids with earlier bedtimes and average-weight parents, the findings showed.
The researchers said that might mean late bedtimes made it more likely that high-risk kids would gain extra weight. Or late bedtimes may simply be part of a general lifestyle that promotes obesity.
But while the cause-and-effect question remains unanswered, the message for parents may still be straightforward.
"An earlier bedtime for kids is absolutely a good idea," Glaser said. Whether that promotes a healthier weight or not, she noted, there are other benefits, including well-rested kids and parents.
"Parents can have some much needed quiet time and time together to recharge the batteries, so they can have more energy for their kids the next day," she said.
Marcus said sleep should be seen as an important element of a healthy lifestyle, along with diet and exercise.
"A well-organized life with good sleeping habits may be of importance, whether it is directly affecting weight or if it is a marker of living habits in general," he said.
Kids, like adults, do vary in how much sleep they need, according to the American Academy of Sleep Medicine. In general, the group recommends toddlers get 11 to 14 hours of sleep each day (naps included), while 3- to 5-year-olds should get 10 to 13 hours.
The American Academy of Pediatrics has more on healthy sleep habits.