Sleep consists of two radically different physiological states. There is rapid eye movement sleep (REM) and non-rapid eye movement sleep (NREM). The sleep stages seem to have different functions, but why we sleep is still not completely understood. Babies spend half of their sleep in REM, but this drops to a quarter by the age of two. It is therefore thought that REM sleep is particularly vital for the developing brain. In NREM sleep, brain activity slows and a person woken at this stage may feel groggy.
During non-REM sleep, the pituitary gland produces growth hormone and secretes prolactin. This counteracts dopamine, to lower general arousal levels.
Your pulse drops by 10-30bpm while you sleep, lowering your blood pressure. Less blood flows to the brain, and more is diverted to your muscles.
You produce less saliva, which reduces the need to swallow. Five per cent of adults also grind their teeth at night, mostly during the early stages of sleep.
The extra blood swells your arms and legs slightly. Muscles are paralysed while dreaming, but between dreams you change sleeping position 35 times a night.
The throat muscles relax so your airway narrows when inhaling. This can cause snoring, or temporarily halt your breathing for a few seconds (sleep apnoea).
Vasopressin hormone levels rise. This reduces the amount of urine collected in the bladder to between a half and a third of normal daytime levels.