Normal sleep
About 60% of the adult population obtain 7–8h sleep per night, with a mean of 7h _[3], but around 8% sleep for less than 5h and 2% for more than 10h (

Fig. 1.3).

The duration of sleep that is obtained often differs considerably from what is needed. For most adults the optimal duration of sleep appears to be 7–8h per night, although many people function almost as well on 6–7h _[4]. Loss of NREM sleep, especially stages 3 and 4, probably causes more daytime sleepiness than loss of REM sleep.

The duration of sleep should be sufficient to lead to a feeling of being refreshed on waking and remaining alert and able to function normally throughout the day _[5]. The rate at which a sleep debt is built up following sleep deprivation is largely genetically determined, but there is little genetic difference in the way that it is relieved by brief naps.

Oversleeping
Epidemiological surveys have shown that long sleepers have a higher mortality than those who sleep for less than 8h each day. Obtaining more sleep than is required, like eating more food than is needed, is of no benefit and can cause problems. Some subjects find it difficult to oversleep, but others can readily do so.

Towards the end of an extended sleep period the duration and depth of NREM sleep begin to increase as REM sleep takes the place of wakefulness in increasing the homeostatic drive to enter NREM sleep. Sleep inertia becomes more prominent and is often prolonged _[6], and the subsequent night’s sleep may become fragmented. Oversleeping should be distinguished from a prolonged sleep to compensate for a sleep debt due to sleep deprivation.

Short and long sleepers
Both short and long sleepers have approximately the same duration of stages 3 and 4 NREM sleep, but short sleepers have less stage 2 and have a shorter sleep latency. They probably have a higher homeostatic drive to enter sleep rather than living with a higher sleep debt. They often require a normal duration of sleep until adolescence, and thereafter, perhaps for social reasons, manage with less sleep than is usual.

Long sleepers have a longer duration of melatonin secretion at night and go to sleep closer to the peak of melatonin secretion. There may be a genetic basis, but the ability to extend sleep beyond physiological needs may be a psychophysiological trait. It is commoner in evening types and is usually associated with a normal sleep latency and sleep efficiency, whereas those who sleep for prolonged periods due to sleep deprivation have a short sleep latency and high sleep efficiency.

References