What is “normal” sleep?

Sleep starts when you first fall asleep and ends when you wake up and stay awake, ready to start your day.

It consists of a number of sleep cycles. The cycles usually occur every 90 to 110 minutes. So someone who sleeps eight hours has around five of these cycles. In turn, each cycle is made up of different stages.

There are two main kinds of sleep:

• REM sleep and
• non-REM (NREM) sleep

REM is short for "rapid eye movement." This name describes the quick eye movements that can be observed in sleeping people. In the REM ("dreaming") stage, children's and adults' eyes move back and forth quickly but their eyelids are still closed.

The period between first falling asleep and entering the next stage of sleep is known as sleep onset. This is a period of very light sleep, where you are drifting in and out of consciousness and are easily woken.

In stage 2, the functioning of the brain slows down but there are still short bursts of activity. This light-sleep stage makes up about half of a total sleep cycle.

Stage 3 is a deep-sleep phase where things like noise don't disturb the person's sleep so easily.

The deep-sleep stage is followed by the REM stage, which is when we dream and our sleep becomes lighter again.

When this stage ends, a new sleep cycle starts. As the night continues, the REM stages become longer and the deep-sleep stages become shorter. Some people completely or partially wake up after a sleep cycle ends, while others stay asleep until morning.

There are plenty of theories about what normal sleep is and how it is affected by age. We know that sleep onset (the time needed to enter into light sleep) increases a little with age. But this age-related difference is very small: Between the ages of 20 and 80, the length of sleep onset increases by less than ten minutes in total.

The REM stages gradually become longer between the ages of 5 and 19. They then stay fairly stable until about the age of 60. People over 60 get slightly less REM sleep, and they don't sleep quite as soundly as younger people do.

For children between the ages of 6 and 12, nine hours of sleep is quite normal. Teenagers typically sleep eight hours. 40-year-olds typically sleep about seven hours, and about 6.5 hours of sleep is the average for people between the ages of 55 and 60. A healthy 80-year-old will usually sleep about six hours a night. But these are all only averages. Everyone needs a different amount of sleep.