NEW YORK - Younger adults who get either little sleep or a lot of it may see a greater expansion in their waistlines over time, a study published on Monday suggests. 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.