While you are sleeping, your body is busy repairing itself. However if you're not getting enough sleep, then your body might not get the recovery and relaxation that it needs in order to repair itself.

 During the first three hours of sleep your body will start producing the human growth hormone from the pituitary gland. As we age, this hormone is necessary for the maintenance of youthful and radiant skin.

In the  middle two hours of sleep is when melatonin is increased. Melatonin is a hormone that is responsible for regulating your circadian rhythm (sleep/wake patterns) but it also acts as an antioxidant, which protects your skin from damaging free radicals.