Functions of Sleep: Restoration Theory

Slow wave sleep (SWS) and REM sleep

Oswald (1980) proposed that SWS enables body repair and REM enables brain recovery.

Growth hormone (GH) is secreted during SWS.

Growth hormone stimulates growth and is therefore particularly important during childhood. It is also important in adulthood because it enables protein synthesis and cell growth to take place. This is vital in the restoration of body tissue because proteins are fragile and must be constantly renewed. This constant restoration of vital proteins is part of the body's natural recovery process.

Sassin et al found that, when sleep-waking cycles are reversed by 12 hours (ie. a person goes to sleep in the morning and gets up at night), the release of GH with sleep is also reversed. This shows that GH release is controlled by neural mechanisms related to SWS.

Further evidence for the link between SWS and GH comes from research which found that the amount of GH released correlates with the amount of SWS (can Cauter and Plat, 1996) and the decline of GH in older age has also been associated with reduced SWS (van Cauter et al, 2000)

Lack of SWS has also been associated with reduced functioning of the immune system - the body's system of defence against viruses and bacteria (Krueger et al, 1985). The immune system consists of various protein molecules - antibodies - which are regenerated during cell growth and protein synthesis in SWS.

This has been explained in terms of their rapid brain growth. It has been suggested that the amount of REM sleep in any species is proportional to the immaturity of the offspring at birth; for example, the platypus is immature at birth and has about 8 hours of REM sleep per day, whereas the dolphin, which can swim from birth, has almost no REM sleep (Siegel et al, 2003)

Siegel and Rogawski suggest that REM sleep allows for a break in neurotransmitter release which in turn permits neurons to regain their sensitivity and allow the body to function properly.

Support for this comes from the action of antidepressant drugs such as MAOIs. These drugs aim to increase the levels of neurotrnasmitters of the monoamine (such as dopamine and seratonin). A side effect is that MAOIs abolish REM activity completely. One suggestion is that these two effects are linked - the increase in monoamines means that monoamine receptors don't have to be revitalised and therefore there is no need for REM sleep.

Crick and Mitchison proposed that during REM sleep, unwanted memories are discarded, thus making more important memories accessible.

Stickgold: the evidence currently suggests that REM may be important in the consolidation of procedural memory (related to skills such as riding a bike), whereas SWS sleep is important for the consolidation of semantic memory (related to knowledge and meaning of things) and episodic memory (memory for events).

The data has tended to be fairly anecdotal and based on individual case studies.

It has suggested that lack of sleep doesn't always result in long term damage and that there is no need to recover anything like the amount of sleep that was lost.

They invariably had short periods of microsleep whilse apparently awake: EEG recordings show that microsleep is the same as sleep (Williams et al, 1959). It could be that apparent 'non-sleepers' are in fact getting the benefits of sleep while appearing to be awake.