Sleep is a very complex process, with many aspects of it still being uncovered by scientists. Chemicals called neurotransmitters are involved in many of the processes that occur during sleep. For example, deep sleep coincides with the release of growth hormones in children and young adults. Another chemical called adenosine builds up in the blood while we are awake and makes us drowsy. It is then gradually broken down while we sleep.

Sleep has five stages €“ 1, 2, 3, 4 and REM (rapid eye movement). These occur in cycles, beginning with stage 1 and moving through to REM, before starting again at stage 1. A complete cycle of all five stages takes between 90 to 110 minutes.

• Stage 1: this is light sleep; we drift in and out of this stage and can be easily awakened
• Stage 2: during this stage, eye movement stops and our brain waves slow. Adults spend almost half of their time asleep in stage 2
• Stages 3 and 4: together these make up what is called 'deep sleep', when it becomes more difficult to wake the person
• REM: during the REM stage, breathing becomes more rapid and irregular and, as the name suggests, the eyes move rapidly. Heart rate and blood pressure also increase and, if a person is woken up, they may recall dreaming. The first REM period occurs around 70 to 90 minutes after falling asleep and is relatively short. As the sleep progresses, REM periods increase in length and deep sleep decreases.

REM sleep stimulates the brain regions used in learning and is thought to be important for normal brain development. This may explain why infants spend about half of their time asleep in the REM stage.