Theories of sleep
Sleep is divided into several different stages. Two of the stages - slow-wave sleep (SWS) and REM sleep - are associated with particular benefits. Oswald (1980) proposed that these each had different functions - that SWS enables body repair and REM sleep enables brain recovery.
Slow-wave sleep (SWS)
Growth hormone is screted during SWS Growth hormone (GH) stimulates growth but also enables protein synthesis, cell growth and tissue regeneration. This is vital because proteins are fragile and must be constantly renewed. GH is secreted in pulses thoughout the day, but a significant amount is released at night and maing during SWS. Sassin et al (1969) found that, when sleep-walking cycles are reversed by 12 hours (i.e. a person goes to sleep in the morning and gets up at night), the release of GH is also reversed. This shows that GH release is controlled by neutral mechanisms related to SWS. Futheer evidence comes from research that found that the amount of GH released correlates with the amount of SWS (van Cauter and Plat 1996).
The immune system Lack of SWS has also been associated with reduced functioning of the immune system, the body's system of defnce 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.
Brain growth The percentage of REM sleep is far higher in babies than adults, and even higher in premature babies. 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 eight hours of REM sleep per day, whereas the dolphin, which can swim from birth, has almost no REM sleep (Seigel 2003). This suggests a relationship between neural development and REM sleep.
Neurotransmitters Siegel and Rogawki (1988) suggest that REM sleep allows for a break in neurotransmitter release which in turn permits neurons to regain their sensitivity and function properly. Support for this comes from the action of some antidepressant drugs such as MAOIs. These drugs aim to increase the levels of monoaime recpetors don't need to be revitalised and therefore there is nood for REM sleep.
REM sleep and memory Research on dreams suggests a link between dreaming and memory. The evidence currently suggests that REM may be important in the consolidation of procedural memory (related to skills such as riding a bicycle) whereas SWS sleep is important for the consoidation of semantic memory (related to knowledge and the meaning of things) and episodic memory (memory for events) (Stickgold 2005).
Warm-blooded animals (like ourselves) need to expend a lot of energy to maintain a constant body temperature. This is particulary problematic for small animals who tend to have high metabolic rates, such as…