Normal sleep and sleep cycles

Definition of sleep

Humans spend almost one-third of life in sleep. Rather than being a period where the brain simply switches off, sleep involves a complex combination of physiological and behavioural processes. Sleep is a state where the senses disengage and we become unresponsive to most of our surrounding environment. It is a time when the body restores itself, our minds organise and categorise memories and learned tasks, and prepare for another prolonged period of complex tasks on waking.

Sleep cycles

  • Sleep normally occurs in cycles lasting an average of 90 minutes
  • Although this does not always occur, each cycle may include transitions from awake to light sleep, deep-sleep and REM (dream) sleep
  • Each sleep-cycle contains a different proportion of Non-REM sleep (non-dreaming sleep - NREM) and REM sleep (dream sleep).
  • More deep sleep occurs at the start of the night, and more REM (dream) sleep occurs in the early hours of the morning
  • Wakefulness usually accounts for 5% or less of the night in most adults.

How much sleep do we need?

On average, humans require about 8 hours of sleep per night, although different people need different amounts of sleep. While some people need only 6 or 7 hours of sleep per night, others may need more - even 10 or 12 hours per night (especially at younger ages).

If deprived of sleep the human body can build up what is called a 'sleep debt' and will try later to "catch up" on sleep in order to reduce the sleep debt. An example of a sleep debt is when you stay up all night and need to sleep for much longer than usual on the following day or night.