What is sleep?
Sleep is a state of consciousness that we spend about one third of our lives in. Sleep cycles through four stages of non-rapid eye movement (NREM) sleep, followed by a period of rapid eye movement (REM) sleep. Stages 1 and 2 of NREM incorporate the process of falling asleep and light sleep, stages 3 and 4 are deep, or "slow wave" sleep.
Broadly speaking, sleep cycles occur repeatedly during the night, in the following order:-
- Stages 1 to 4,
- Back through stages 3 and 2, and
- Into REM sleep, during which we have our most vivid dreams.
- Then it cycles back to stages 2, 3 and 4 and repeats the cycle.
After we have been asleep for some time, the pattern alters. REM sleep increases and slow wave sleep decreases. The amount of time spent in each stage can vary between individuals and is particularly affected by advancing age. The night-long individual pattern is frequently referred to as "sleep architecture" (Goetz & Pappert, 1999; Morin & Espie, 2003).
Polysomnography is the study of sleep, which records activity of the brain and muscles (including the heart), eye movements and breathing efficiency.
Why do we sleep?
The precise function of sleep is the subject of ongoing research. We know that sleep is essential to our wellbeing and there is evidence that animals suffering prolonged total sleep deprivation will eventually die. Beyond that, it is thought that sleep probably restores the body and brain, it may be important for the conservation of energy or to keep us safe during darkness. It is known, for example, that certain metabolic changes and hormonal activity occur during, or primarily during, sleep, but it is thought that these functions could also take place in the waking state (Goetz & Pappert, 1999).
We do know, however, that sleep deprivation has a range of effects. The most obvious is fatigue or sleepiness, which causes changes in mood and temperament, impairment of concentration, memory, and learning, and feelings of confusion (Morin & Espie, 2003). More recently, studies have shown links between insomnia and a long list of medical disorders (Budhiraja, Roth, et al., 2011).
For more details, see the Wikipedia entry for Sleep Deprivation.