There are of course many lifestyle factors that determine who becomes overweight and who does not, such as food intake, genetics and stress. But sleep is more important in the process than most people realize.
I love this photo of someone on an exercise machine falling asleep. Exercise is great AND you need your full restorative sleep to balance your hormones or a cascade can start up that results in weight gain. This cascade involves cortisol and leptin and ghrelin.
The hormone Leptin is involved in appetite, fat burning and metabolism. It is the chemical that tells your brain when you are full and when you should start burning calories. During sleep, leptin levels rise and your hunger level goes down.The decrease in leptin brought on by sleep deprivation can result in a constant feeling of hunger and a general slow-down of your metabolism.
The other hormone found to be related to sleep and weight is GHRELIN. The purpose of ghrelin is basically the exact opposite of leptin: It tells your brain when you need to eat, when it should stop burning calories and when it should store energy as fat. During sleep, levels of ghrelin decrease, because sleep requires far less energy than being awake does. People who don’t sleep enough end up with too much ghrelin in their system, so the body thinks it’s hungry and it needs more calories, and it stops burning those calories because it thinks there’s a shortage.