Sleep physiology The cycle between sleep and wakefulness involves different stages of sleep. Currently, scientists divide sleep into two general types: REM (Rapid Eye Movement) and NREM (non-REM). REM sleep is characterized by desynchronization of the electroencephalograph (EEG), loss of skeletal muscle tone, and sympathetic nervous system activity; whereas non-rapid eye movement sleep is characterized by parasympathetic nervous system activity (Legramante & Galante, 2005). http://upload.wikimedia.org/wikipedia/en/thumb/4/43/SWS.jpg/165px-SWS.jpg http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.pngStage 4 Sleep. EEG highlighted by red box. http://upload.wikimedia.org/wikipedia/en/thumb/4/44/REM.png/165px-REM.png http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.pngREM Sleep. EEG highlighted by red box. Eye movements highlighted by red line. Non-REM accounts for 75?80% of total sleep time, and consists of four stages: Stage 1, with near-disappearance of the alpha waves seen in awake states, and appearance for the first time of theta waves. The stage is sometimes referred to as somnolence,or "drowsy sleep". It appears at sleep onset (as it is mostly a transition state into Stage 2) and is associated with the sudden twitches or hypnic jerks many people experience when falling asleep. While these are normal and of no concern, the hypnagogic hallucinations which some people may experience at this stage can be more troublesome. During this period, the subject loses some muscle tone, and conscious awareness of the external environment: Stage 1 can be thought of as a gateway state between wake and sleep. Stage 2, with "sleep spindles" (12?16 Hz) and "K-complexes." The EMG lowers, and conscious awareness of the external environment disappears. This occupies 45?55% of total sleep.Stage 3, with delta waves, also called delta rhythms (.5?4 Hz), is considered part of slow-wave sleep (SWS) and functions primarily as a transition into stage four. Overall it occupies 3?8% of total sleep time.Stage 4 is true delta sleep. It predominates the first thirdof the night and accounts for 10?15% of total sleep time. This is often described as the deepest stage of sleep; it is exceedingly difficult to wake a subject in this state. This is the stage in which night terrors, bed wetting, sleepwalking, and sleep-talking occur. REM sleep is popularly associated with dreaming, especially bizarre, visual, and seemingly random dreams; however, dreams can also occur during sleep onset (hypnogogia) and during all stages of Non-REM sleep.[4] REM sleep is predominant in the final third of a sleep period; its timing is linked to circadian rhythm and body temperature. The EEG in this period is aroused and looks similar to stage 1, and sometimes includes beta waves. Also known as Stage 5 sleep.Active Sleep is a phase of sleep in neonates that appears similar to Rapid Eye Movement(REM) sleep in adults. While it depends on age, neonatal sleep is sometimes scored as Active Sleep, Quiet Sleep, and Wake. This is less specific than the classification of adult's sleep, and is often based on behavioral criteria due to the technical difficulties arising from recording EEG from the neonate. Scientists are divided on the precise relation between Active Sleep and REM sleep. Some suggest that they are similar, while others say it is an entirely different state, which represents aspects of the developing CNS that are not present in a mature brain, and that certain aspects of REM are not present in Active Sleep due to the immaturity of the CNS in the neonate. Sleep proceeds in cycles of NREM and REM phases. In humans, the cycle of REM and NREM is approximately 90 minutes. Each stage may have a distinct physiological function. Drugs such as alcohol and sleeping pills can suppress certain stages of sleep (see Sleep deprivation). This can result in a sleep that exhibits loss of consciousness but does not fulfill its physiological functions. Each sleep stage is not necessarily uniform. Within a given stage, a cyclical alternating pattern may be observed. Both REM sleep and NREM sleep stages 3 and 4 are homeostatically driven; that is, selective deprivation of each of these states subsequently causes a rebound in their appearance once the person is allowed to sleep. This finding leads to the ubiquitous assumption that both are essential in the sleep process and its many functions. REM sleep may also be driven by a circadian oscillator, as studies have shown that REM is temporally coupled with the circadian rhythm of temperature. ....................from Wikipedia
