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1. 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.

2. 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.)

3. 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.

4. 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.

5. In fact, this phenomenon is the result of metabolic
dysfunction due to short sleep duration involving
hormones ghrelin and leptin.

6. 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.

7. 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.

8. 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.

9. Thus decrease in the amount of leptin after sleep
deprivation would increase caloric intake while at the
same time, decreases energy expenditure.

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

11. 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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