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"Disturbed sleep resulted in a 27 percent average decrease in insulin secretion after eating, and higher glucose levels over a longer period of time, sometimes high enough to make the subject prediabetic. In addition, there was an average 8 percent decrease in resting metabolism rate, a measure of how much energy the body consumes at rest, that translates into a theoretical weight gain of more than 12 pounds a year."
Previous studies also linked interrupted or poor sleep to higher cortisol blood levels, which was thought to explain some of the weight gain. It has also been speculated that those who keep late hours to go to sleep tend to overeat at night. But this study actually measured insulin response and blood sugar in those who had interrupted sleep and saw a dramatic decrease in insulin's effectiveness at controlling blood sugar.
So, to keep from gaining weight, it is not only important to eat healthy and exercise, but to try to get a full night sleep every night.