According to a new study women who sleep 5 hours or less per night weigh more on average than those who sleep 7 hours.

Researchers from Ohio's Case Western Reserve University, followed nearly 70,000 women for 16 years and found that women who slept five or fewer hours a night were a third more likely to put on at least 33lbs (15kg) than sound sleepers during that time.

The study included 68,183 middle-aged women who were enrolled in the Nurses Health Study who were asked in 1986 about their typical night's sleep, and were then asked to report their weight every 2 years for 16 years.

On average, women who slept 5 hours or less per night weighed 5.4 pounds more at the beginning of the study than those sleeping 7 hours and gained an additional 1.6 pounds more over the next 10 years.

Lead researcher Sanjay Patel, M.D., Assistant Professor of Medicine, says while that may not sound very much, it is an average amount and some women gained much more than that.

Patel says even a small difference in weight can increase a person's risk of health problems such as diabetes and hypertension.

Dr. Patel says the study is by far the largest study to track the effect of sleep habits on weight gain over time and is the first to show reduced sleep increases the risk of gaining weight over time.

The researchers looked at the women's diets and exercise habits to see if they could account for part of the findings but found that appetite and diet do not account for the weight gain in women who sleep less and they found little difference in physical activity that could explain why women who slept less weighed more.

Dr. Patel suggests that sleeping less may affect changes in a person's basal metabolic rate and non-exercise associated thermogenesis, or NEAT, which refers to involuntary activity, such as fidgeting or standing instead of sitting.

It may be says Patel that if you sleep less, you move around around less too, and therefore burn up fewer calories.

The research was presented at the American Thoracic Society International Conference on May 23rd.

Dr. Tashkin say the new findings are surprising for several reasons; previous studies have shown that marijuana tar contains about 50% higher concentrations of chemicals linked to lung cancer, compared with tobacco tar, and smoking marijuana cigarettes deposits four times more tar in the lungs than smoking an equivalent amount of tobacco.

Apparently marijuana is packed more loosely than tobacco, so less filtration takes place through the rod of the cigarette, so more particles are inhaled.

Dr. Tashkin says also that marijuana smokers hold their breath about four times longer than tobacco smokers, allowing more time for extra fine particles to be deposited in the lungs.

Tashkin does offer one possible explanation in that tetrahydrocannabinol (THC), a chemical in marijuana smoke, may encourage aging cells to die earlier and therefore be less likely to undergo cancerous transformation.

Dr. Tashkin says, the next step is to study the DNA samples of the subjects, to see whether there are some heavy marijuana users who may be at increased risk of developing cancer if they have a genetic susceptibility for cancer.

Other experts are warning that the study should not be viewed as a green light to smoke pot, as smoking marijuana has been associated with problems such as cognitive impairment and chronic bronchitis.