What happens to you while you sleep

Body temperature: Most body systems wind down during sleep, the heart rate drops and so too does the body temperature and metabolic rate.

Muscles: They get repaired during deep sleep. Researchers found that people who had spent the previous waking hours in hard physical activity spent more time in deep sleep.

Skin: Some research shows that during sleep skin becomes dehydrated. A glass of water at night or night creams can counteract the effects. Lack of sleep also affects the skin, possibly because it is not getting repaired by nocturnal growth hormones. Black rings in the thin skin around the eyes are the first indication of sleep deprivation.

Behaviour: Emotions are heavily influenced by the amount of sleep, probably because the cerebral cortex, the part of the brain involved, can't wind down during the day and needs to take full advantage of sleep time. Irritability, poor concentration, aggression, apathy, disorientation and paranoia are the consequences of sleep loss.

Learning: Although we pick up new things during the day, the learning process takes place during the following eight hours while we sleep. Sleep before an exam, for example, has been found to be as important as revision the night before. Researchers found that people who don't sleep after revising are less likely to remember.