Do you notice that you're hungrier when you sleep less?? It's not your imagination...
The hormone leptin is important for the regulation of appetite and metabolism. Leptin is the chemical that tells your brain when you're full, as well as when to use food for energy. It triggers a cascade starting in the hypothalamus and ending in the thyroid gland. The thyroid gland controls the way your body stores and uses energy.
During sleep, leptin levels increase. When you don't get enough sleep, leptin decreases and your brain thinks it needs more calories. So you feel hungry, even though you don't actually need food at that time. You ingest calories, and your body stores them for energy. The decrease in leptin caused by sleep deprivation can result in a constant feeling of hunger and a general slow-down of your metabolism.
The other culprit? Ghrelin.
Ghrelin is the exact opposite of leptin. Ghrelin tells your brain when you need calories, when to stop burning calories, and when to store calories. During sleep, ghrelin decreases, because sleep requires less energy. If you don't sleep enough, ghrelin accumulates in your system. You feel hungry, and you stop burning calories because your body thinks it is in "starvation mode".
Sleep deprivation has also been found to increase levels of cortisol and insulin resistance, both of which also contribute to weight gain.
The Doctor's Orders
You should aim for at least 7 1/2 hours of sleep each night. The goal is to hit 5 REM (rapid eye movement) cycles. These cycles occur every 90 minutes. So stop reading this and go to sleep!