From 8pm to 11pm is the time for hydration and stimulation, while 11pm to 3am is the time for nutrition and regeneration, and 3am to 5am is the time for resting.
Of course, very few adults are in bed at 8pm and, as the hormone that triggers sleep - melatonin - peaks at around 11pm, this is the time we should naturally get tired.
Seven or eight hours of sleep is ideal for most of us. More than that can mean we wake up looking puffy and feeling sluggish.
Chronic lack of sleep, or poor-quality sleep, has an incredibly negative effect on the way we feel - and on the way we look.
When we are asleep, our cells rebuild and repair themselves (the growth hormone functions only at night). If you don't sleep, this function is impaired.
If you sleep badly, you are likely to become stressed, and this can cause the capillaries to tighten up, affecting the flow of nutrients to the skin and scalp and causing the skin and hair to look dull.