Sleeping too little or too much will make you fat.

From New Scientist:

Sleep's effect on fat is becoming clearer. Having too much or too little piles on the worst kind of fat.

Kristen Hairston and colleagues at Wake Forest University in Winston-Salem, North Carolina, monitored 1100 African and Hispanic Americans for five years. Both groups are at a high risk of obesity-related disorders.

People under 40 gained 1.8 kilograms more on average if they got less than 5 hours of sleep per night than if they slept for 6 or 7 hours. Those regularly sleeping for more than 8 hours gained 0.8 kilograms more than the medium-sleep group (Sleep, vol 33, p 289).

CAT scans revealed increases in visceral fat, which accumulates around the internal organs and is particularly dangerous to health.