Researchers found that among black and Hispanic adults younger than 40, those who typically slept for five hours or less each night had a greater accumulation of belly fat over the next five years, versus those who averaged six or seven hours.
Those who logged eight hours or more in bed each night also showed a bigger fat gain -- but it was less substantial than that seen in "short sleepers."
The study, reported in the journal Sleep, does not prove that too little or too much sleep directly leads to excess fat gain. But the findings support and extend those of other studies linking sleep duration -- particularly a lack of sleep -- to weight gain and even to higher risks of diabetes and heart disease.
The study adds to past research in part because it focused on black and Hispanic Americans -- two understudied groups who are at increased risk of obesity and its related ills, said lead researcher Dr. Kristen G. Hairston, of Wake Forest University School of Medicine in Winston-Salem, North Carolina.
It also zeroed in on the relationship between sleep and gains in abdominal fat -- both the superficial fat layers just below the skin and the "visceral" fat that surrounds the abdominal organs. Deep abdominal fat is believed to be particularly important in the risks of health conditions like diabetes, high blood pressure and heart disease, Hairston told Reuters Health.
The study included 332 African-American and 775 Hispanic-American men and women ages 18 to 81. At the outset, all reported on their sleep habits, diets, exercise levels and other lifestyle factors. The researchers used CT scans to measure participants' abdominal fat, at the start of the study and again five years later.
Among participants younger than 40, the study found, those who said they slept for five hours or less each night gained more belly fat than those who averaged six or seven hours of sleep.
On average, short sleepers showed a 32 percent gain in visceral fat, versus a 13 percent gain among those who slept six or seven hours per night, and a 22 percent increase among men and women who got at least eight hours of sleep each night.
A similar pattern was seen with superficial abdominal fat. Even when the researchers considered factors like calorie intake, exercise habits, education and smoking, sleep duration itself remained linked to abdominal-fat gain.
The findings, according to Hairston, support the belief that sleep habits affect weight, and health in general. "Sleep is an important part of your overall health -- not just in whether you're tired during the day," she said.
Individuals vary in their sleep needs, so there is no one set prescription. But "extremes of sleep," such as less than five hours per night, should raise concerns, according to Hairston. "And if you're concerned about your sleep," she said, "discuss it with your healthcare provider, just as you would discuss diet or exercise."
As for why sleep duration might affect abdominal-fat gain, there are several theories.
There may be indirect effects; people who get too little sleep may be too tired during the day to exercise, while those who spend a lot of time in bed may spend less time being active, relative to people who sleep fewer hours.
Research also suggests that sleep loss alters people's levels of appetite-regulating hormones -- which could, in theory, spur them to overeat.
Depression, which often affects people's sleep and has been linked to weight gain, could also be a factor, Hairston noted. She and her colleagues had no information on study participants' depression symptoms.