1. Sleep slows down the skin's ageing process.
2. Blood flow to your skin increases at night so your beauty products are absorbed more quickly.
3. Your skin rehydrates while you are asleep which decreases dark circles and fine lines.
4. Skin conditions like acne and eczema are linked to stress - while you're asleep you rest and relax so these conditions can improve.
5. During the day your skin is exposed to pollutants and ultraviolet radiation but gets a break from these at night while you're asleep.
Source: http://www.webmd.com/beauty/skin/beauty-sleep_