Sleep is a behavioural state – we spend our lives either awake or asleep.  Although all of sleep appears as a single state, it is actually comprised of two different types of sleep. These are known as Rapid Eye Movement Sleep (REM Sleep for short) and Non-Rapid Eye Movement Sleep (NREM sleep – pronounced ‘non-REM’ sleep).  NREM sleep is divided into three stages of increasing depth of unresponsiveness, known as N1, N2 and N3 sleep. N3 sleep is also known as “deep sleep” or “slow-wave sleep”.

Through the night, REMsleep and NREM sleep alternate.  A period of NREM sleep is followed by a period of REM sleep. Together this constitutes a “sleep cycle”. The duration of REM sleep in the first sleep cycle is quite short and in successive sleep cycles it lengthens as the night progresses. Therefore much of the REM sleep occurs in the latter part of the night.

In adults, a night’s sleep consists of between four and six sequential cycles of sleep each lasting approximately 90 minutes. The following diagram is known as a ‘hypnogram’ and gives a visual summary of a night’s sleep:

How much sleep do you need?

Sleep duration is determined by your genes.  The average sleep requirement varies considerably between different individuals. Actually, it varies between four and 11 hours!!  That said, approximately 2/3 of the general population require between 7 and 8 hours of sleep.

Once we turn off the light and decide to sleep, it takes a few minutes for us to fall asleep.  The time taken to sleep is known as Sleep Latency. Usually sleep latency is less than 30 minutes.   It is common to wake up a few times during the night.  Good sleepers spend only a few minutes awake and return to sleep quickly.  Others spend a little longer, but on average people do not spend more than 30 minutes awake after first falling asleep. On average the time spent awake after first falling  asleep is less than 30 minutes.

Sleep satisfaction:

It is usual to hear experts and talk-show hosts talking about ‘good quality’ sleep.  On the one hand we all know when we have had a good night’s, satisfying sleep.  However, if you are asked as to what is meant by ‘good quality’ sleep, it is difficult to answer.  In fact, several factors combine together to produce satisfying sleep. These are:

1. Sleep Duration: we need to get as much sleep as we need (see above).
2. Sleep Continuity: is the second most important factor.  If you wake up several times and spend long times awake in bed, then sleep is unlikely to be satisfying  and you are likely to report ‘poor quality sleep’.
3. Sleep efficiency: this is the percentage of time we spend sleeping while we are in bed. For example, if you spend 10 hours in bed at night and sleep for only 8 hours out of it, the sleep efficiency is  80%  (i.e., Time spent sleeping/time in bed multiplied by 100).  A sleep efficiency of <85% indicates poor quality sleep.  People with sleep efficiency of  90 % or more are likely to feel they have good quality sleep.
4. Sleep timing: we are meant to sleep at night and keep awake in the day time.  If this is not possible due to family, social or work reasons, it is unlikely that you will have the high quality sleep.