Sleep is made up of a number of stages, known as stages one, two, three and four and rapid eye movement (REM) sleep.
During stage one, you are in transition between being awake and asleep, and wake easily. During stages two, three and four, your eye movements stops, your body temperature falls, and you are deep asleep. In REM sleep, your eyes move rapidly, your blood pressure and heart rate go up, and your brain becomes very active.
REM sleep is when most dreaming happens, and is thought to be important for learning and creating new memories. You spend about a fifth of your sleeping time in REM sleep.
The sleep cycle
Sleep is not only made up of different stages, but also follows cycles. About every one-and-a-half to two hours, a new cycle of sleep begins. Each cycle includes some REM sleep, as well as very brief periods of wakefulness, which you are usually not aware of.
The first couple of cycles contain relatively short periods of REM sleep and long periods of deep sleep (stages three and four). As your sleep progresses, you spend more time in REM sleep and less in deep sleep. Of all the sleep you have, the first three hours are often the deepest – this is called ‘slow wave sleep’.
Last reviewed: November 2016