Sleep has been described by psychologists as an altered state of consciousness (Mavanji et al, 2011), and is viewed by Greene and Siegel (2004), as a period of rest by during which an individuals experience declines in consciousness, motor output and responsiveness to stimuli from their surrounding environment. Sleep is often believed to be an important aspect of daily functioning, a viewpoint most likely originating from the fact that the average person will spend almost a third of their life in a state of sleep. Just how important it is however, has proved to be a difficult question to answer and even after decades of research and the proposal of various theories, we still cannot say that we have reached a widely accepted conclusion as to the main function of sleep (Zepelin, Siegel and Tobler, 2005), with some researchers describing sleep as "one of the major unanswered questions in biology" (Bennington & Heller, 1995). The purpose of this essay then, will be to firstly outline the characteristics and stages of sleep and secondly, compare and evaluate a number of popular theories which attempt to explain is purpose.
To begin with, it is important to introduce the characteristics of sleep and how these properties are measured by researchers. The physiological features of sleep can be measured using a number of methods designed to record electrical activity during sleep, including the electromyogram, which measures eye movements during sleep and the electroencephalogram (EEG), used to measure neuronal activity in the brain. The most commonly used, the EEG, measures electrical activity being emitted from neurons within the brain and has proven to be an effective tool in enabling researchers to successfully differentiate between states of wakefulness and the various stages of sleep by analysing the marked changes in the electrographic waveform associated with each. .