This seems to be a simple question, but the answer is, unfortunately, complex. Sleep is often described as ‘a normal, recurrent period of rest of the organism, accompanied by a lowering of the level of consciousness and, as a result of this, a state of being relatively closed off from the outside world’.

The duration of sleep

The mean duration of sleep is about 8 hours per night. At birth this is over 15 hours and decreases until the age of 15, after which it remains somewhat constant, apart from a slight decrease towards old age. It is interesting to see that after the age of 35 the time one spends in bed increases, while the duration of sleep shows a slight decrease. This increases the chance of experiencing problems sleeping. The variability in the duration of sleep between individuals is even more pronounced when ageing. Some of the elderly sleep report their sleep quality to be the same as young adults, but generally speaking there are more complaints about sleep.

So, there is not only a lot of variability in the duration of sleep with increasing age, there are also big differences between individuals. In the absence of disturbed sleep or a sleep disorder, one can distinguish short-sleepers and long-sleepers. Examples of people that needed extremely little sleep are Napoleon and Churchill. An example of the opposite is Einstein: a real long-sleeper.
This means that it is not possible to say what the absolute minimum (or maximum) amount of sleep is that an individual needs. Whether or not the amount of sleep is sufficient for a certain individual is usually indicated by the day-time functioning. If there are no complaints in the day-time functioning, it is assumed that the amount of sleep during the night is sufficient.

Sleep phases

Besides the 24 hour cycle of waking and sleeping, different sleep-phases can be distinguished. These sleep-phases show a cycle of about 90 minutes and within these cycles a number of sleep-phases occur.
Sleep onset is a period of drowsiness. In this transitory phase sleep is light and on is easily aroused. The phase of light sleep, phase 2, is the first ‘real’ sleep phase and lasts about 10 to 20 minutes. When aroused from this phase people usually report sleep. After this phase the deep sleep phases follow. People are less easily aroused. After this sleep becomes more shallow and the first period of REM (rapid eye movement) sleep begins, a sleep phase characterised by vivid dreams.
As the night progresses the amount of deep sleep decreases and the amount of REM sleep increases. This means that awakening is more frequent in the later parts of the night, when sleep is more shallow and more dreaming occurs.

The way sleep is structured (the amount of time in the several sleep-phases, etc.) shows the same variation between individuals as the duration of sleep. An important factor in the structure of sleep is age, in the same way it is in the duration of sleep. In new borns deep sleep is about half of the total sleep time, the other half being filled with REM sleep. In adolescence this is 17% and 25% respectively. With increasing age the amount of deep sleep further decreases.

The function of sleep

As to the exact function of sleep much still remains unclear. Most of the theories on the function of sleep refer to the restorative aspect of it. In these theories deep sleep is mostly regarded as being important for physical restoration. REM sleep is often seen as important for emotional recuperation.