NEW YORK - Women who fail to get enough shut-eye each night risk gaining weight, a Cleveland-based researcher reported at a medical conference in San Diego Wednesday.

In a long-term study of middle-aged women, those who slept 5 hours or less each night were 32 percent more likely to gain a significant amount of weight (adding 33 pounds or more) and 15 percent more likely to become obese during 16 years of follow-up than women who slept 7 hours each night.

A bed in a hotel in this undated photo. [Reuters]

This level of weight gain -- 15 kg, or 33 pounds -- is "very clinically significant in terms of risk of diabetes and heart disease," Dr. Sanjay Patel of Case Western Reserve University told Reuters Health.

Women who slept 6 hours nightly were 12 percent more likely to experience major weight gain and 6 percent more likely to become obese compared with those who slept 7 hours each night.

The 68,183 women in the study provided information in 1986 on their typical night's sleep and reported their weight every 2 years for 16 years. The findings were presented at the American Thoracic Society's International Conference.

Women who said they slept for 5 hours or less each night, on average, weighed 5.4 pounds more at the beginning of the study than those sleeping 7 hours.

After accounting for the influence of age and weight at the beginning of the study, women who slept 5 hours or less each night gained about 2.3 pounds more during follow-up than those who slept 7 hours nightly. Women who got 6 hours of shut-eye each night gained 1.5 pounds more than those who slept 7 hours nightly.

The researchers analyzed the diets and physical activity levels of the women, but failed to find any differences that could explain why women who slept less weighed more. "We actually found that women who slept less, ate less," Patel said.

"In terms of exercise, we saw a small difference in that women who slept less exercised slightly less than women who slept more but it didn't explain the magnitude of our findings," Patel said.

All in all, it seems that diet and exercise are not accounting for the weight gain in women who sleep less, Patel concluded.

It's possible that sleeping less may affect changes in a person's basal metabolic rate -- the number of calories burned when at rest, Patel said.

Another possible contributor to weight regulation that's come to light recently is called "non-exercise associated thermogenesis" or NEAT, which refers to involuntary activity such as fidgeting or standing instead of sitting. It may be, Patel said, that if people who sleep less, also move around or "fidget" less.