Large-scale study from the National Center for Health Statistics finds people who sleep fewer than six hours a night had the highest rates of obesity. Sleeping more than nine hours a night was also linked with higher body mass index.

People who got more or less than the average seven to eight hours of shut-eye a night were also more likely to smoke, drink five or more alcoholic beverages a day, and lead sedentary lives.

What's unclear is which comes first poor sleep, or obesity and these other lifestyle habits.

Obesity may also be linked with the risk for Alzheimer's disease and other forms of dementia.

A review of research suggests obesity increases the relative risk of dementia for both men and women by about 42 percent when compared with adults at a normal weight.

When researchers looked specifically at Alzheimer's disease, the risk shot up to 80 percent.

It wasn't just weight gain that affected the risk. Those who were underweight were also more likely to develop dementia.