According to many large scale studies conducted in different parts of the world (including Japan, Australia,

Italy and the U.S.), people sleep less will have higher probability of being obese.

This relationship was revealed in 18 studies with total sample size of 604,509 adults sleeping less than 5 hours. There was also a dose effect of sleep duration: for each

additional hour of sleep, people's BMI would decrease by 0.35 kg/m2. (Body mass index (BMI) is a simple index of weight-for-height that is commonly used in classifying obesity and overweight. It is defined as the weight in

kilograms divided by the square of the height in meters (kg/m2). The World Health Organization defines

"overweight" as a BMI value equal to or more than 25, while for "obesity", the value is 30.)

For example, in Japan, a study of 35,247 workers over 1 year found that short sleep duration (having 6 hours or less) was associated with an increased risk of obesity in

men.

Similar findings were observed in the American and Australian studies (with sample size of 56,507 and 45,325 adults respectively). In Italy, a 6-year study of 1,597 male

and female adults shows that every additional hour of sleep would decrease the incidence of obesity by 30 per

cent.

In fact, this phenomenon is the result of metabolic dysfunction due to short sleep duration involving

hormones ghrelin and leptin.

Ghrelin is a hormone which promotes hunger and stimulates gastric emptying. But its amount increases with sleep restriction. Thus you will eat more when you sleep less. Additionally, it suppresses fat utilization in adipose tissue, Overall, ghrelin is one of several hormonal signals

that communicates the state of energy balance in the body to the brain.

On the other hand, another hormone, leptin, contributes to satiety perception, decreases with short sleep duration.

Thus the result is the same: you want to eat more when you sleep less.

In addition, less sleep could affect energy balance by decreasing energy expenditure. Leptin has a central role in fat metabolism, as a signal to the brain that there is fat on

the body. And so it increases energy expenditure. With less amount of leptin because of sleep less, energy

expenditure would decrease.

Thus decrease in the amount of leptin after sleep deprivation would increase caloric intake while at the

same time, decreases energy expenditure.

Excessive food intake associated with insufficient sleep may be a mechanism for increased obesity risk.

Being overweight or obese is one of the main risk factors for obstructive sleep apnea (OSA, a condition in which the

flow of air pauses or decreases during breathing while asleep). But people suffering from more severe OSA in turn are more likely to gain more weight. The effects of

OSA itself may predispose people to weight gain.

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