For years, scientists have known that sleep is a vital part of overall body rejuvenation: When you enter the REM part of the sleep cycle, cells throughout the body repair and restore themselves. This happens in your skin cells, too.
When you sleep, your body releases hormones that have loads of beauty benefits. Notably, melatonin. Melatonin is critically important for your skin and plays a significant role in skin repair from environmental exposure, UV light, stressors, pollution, if you're not resting - you're not generating that melatonin, and your skin isn't repairing itself.