Researchers followed 68,183 middle-aged women for 16 years and found that those who slept 5 hours or less per night were one third more likely to gain weight than those who slept for 7 hours.

Moreover, researchers found that the weight gain was substantial with some women even gaining 33 pounds or more.

The associations between sleep duration and weight gain persisted even after controlling for factors such as physical activity and calorie consumption in both groups.

The findings, published in the American Journal of Epidemiology and presented earlier this year at a medical conference, furthers the evidence that sleep habits affect a person's weight.

The exact reasons for association between sleep duration and weight gain aren't clear but some research suggests that sleep deprivation alters hormones involved in appetite control and metabolism.

Also it's possible that people who sleep fewer hours either eat more or, because of fatigue, and exercise less often.

The research was led by Dr. Sanjay R. Patel of Case Western Reserve University in Cleveland.

The participating women reported their habitual sleep duration in 1986 in the Nurses' Health Study, which has followed the health of thousands of female nurses for the past 30 years.