Previous research has suggested insufficient sleep before age 4 raised the risk of obesity. But the new study, published online May 19 in Pediatrics, observed the link from infancy to mid-childhood.
"Insufficient sleep is an independent and strong risk factor for childhood obesity and the accumulation of total fat and abdominal fat," said study researcher Dr. Elsie Taveras, chief of general pediatrics at Massachusetts General Hospital for Children in Boston.
"The main strength of this study is we looked at sleep at multiple periods," she added.
Excess body fat in childhood sets the stage for serious health problems, such as heart disease and diabetes.
Taveras and her team studied more than 1,000 children. Curtailed sleep was defined as fewer than 12 hours a day from ages 6 months to 2 years, fewer than 10 hours daily for ages 3 and 4, and fewer than nine hours a day for ages 5 to 7.
Kids who were most sleep deprived were about 2.5 times more likely to be obese than those who slept the most, the study found.
They were also 2.5 times more likely to have higher total fat, higher abdominal fat and a higher waist and hip circumference, said Taveras.
Many possible explanations exist for this association, Taveras said.
"If you sleep too little, it disrupts some of the hormones that regulate how hungry we are and how full we are," Taveras said.
In households with no consistent bedtime for children, there is likely to be chaos around regular mealtimes, too, and that can affect weight, she added.
Children who don't sleep enough may watch more television than kids who go to bed earlier, she said. Watching TV has long been linked with eating more, especially in response to food commercials, she said.
Or the children may have other "high-tech distractions," she said.
The new findings don't surprise Dr. William Muinos, director of the weight management program at Miami Children's Hospital in Florida, who was not involved in the study.
His advice for parents? Set a consistent bedtime. "Limit caffeinated beverages late in the day. Cut out all those electronic distractions; get them out of the bedroom," Taveras said.
Insist that children go to bed earlier, Muinos said. "Lack of sleep does change the physiology," he added. "It will put you in what is called stress mode. The body will read it as, 'I need to hold onto calories and accumulate fat.'"
The new research, Muinos said, "is very good because it studied a large group of children for a long period of time."
Ongoing studies are looking at whether improving sleep may directly improve weight control in children, Taveras said. And it's already known that good sleep has other benefits. "There's really good evidence that shows it improves schoolwork," she said.
For the study, mothers reported their children's usual sleep duration in a 24-hour period, beginning at age 6 months. They also reported it every year from age 1 to 7.
The children got a sleep score, ranging from zero (insufficient sleep) to 13 (consistent sufficient sleep). The average sleep score was 10.2. However, about 4 percent of kids were in the insufficient range, zero to 4. About 40 percent had a score of 12 or 13, termed enough sleep.
Those who slept less than average were more likely to be from poorer families with less educated mothers, the study found.
To learn more about sleep, visit the National Sleep Foundation.