Physiological changes that occur during sleep have been well documented, and researchers have been interested in the mechanisms that bring about these changes. Another area of interest has been the question of why we need to sleep, and a number of explanations have been put forward. A further question is how sleep changes across the lifespan.
The nature of sleep – sleep stages
While the cycle of sleeping and waking has a circadian rhythm, there are also stages within sleep that are based on a 90-minute ultraradian cycle. EEGs or electroencephalographs are often used to record sleep stages as they measure electrical activity in different parts of the brain.
One particular kind of sleep that has been of interest to psychologists is rapid eye movement (REM) sleep. People experience several periods of this in the course of a night’s sleep, during which the eyes make rapid movements that produce intense activity. In an early study, Dement and Kleitman (1957) found that REM sleep shows a distinctive pattern of brain activity and that it is predominantly associated with dreaming. Following on from this, the different kinds of sleep we experience have been categorised by Rechtschaffen and Kales (1968).
According to them there are 5 stages of sleep:
Stage 0: Wakefulness, this is identified by the presence of low-amplitude, high frequency beta waves in the EEG. As we relax, these are replaced by alpha waves, which are higher in amplitude but lower in frequency (8-12 cycles per second).
Stage 1: This is characterised by the appearance of theta waves in the EEG; these are slower (4-7 cycles per second) and more…