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Non-REM sleep, which is perhaps best defined negatively as any sleep not recognizable as REM sleep, consists of three separate stages (stage1, stage 2 and stage 3), which are followed in order upwards and downwards as sleep cycles progress. Formerly, four stages of non-REM sleep were distinguished, and most older hypnograms therefore usually show four stages of non-REM sleep, rather than three; the distinction can be quite useful at times, and is still quite widely used, even though three stages is now the “official” categorization. It should be noted that the distinctions between these sleep stages are somewhat arbitrary anyway, and the physiological boundaries between them are necessarily blurred and continuous.
Stage 1 (NREM1 or N1) is the stage between wakefulness and sleep, sometimes referred to as somnolence or drowsy sleep, in which the muscles are still quite active and the eyes roll around slowly and may open and close from time to time. In more scientific terms, stage 1 is the period of transition from relatively unsynchronized beta and gamma brain waves (with a frequency of 12-30 Hz and 25-100 Hz respectively), which is the normal range for the awake state, to more synchronized but slower alpha waves with a frequency of 8-13 Hz, and then to theta waves with a frequency of 4-7 Hz. It is difficult to pinpoint the actual point of sleep onset (falling asleep), as the process is a continuum as brain wave activity gradually slows down.
Stage 2 (NREM2 or N2) is the first unequivocal stage of sleep, during which muscle activity decreases still further and conscious awareness of the outside world begins to fade completely. If any sounds are heard, the sleeper is not able to understand their content at this point. Brain waves during stage 2 are mainly in the theta wave range (as in stage 1 sleep), but in addition stage 2 is also characterized by two distinguishing phenomena: sleep spindles (short bursts of brain activity in the region of 12-14 Hz, lasting maybe half a second each, also known as sigma waves) and K-complexes (short negative high voltage peaks, followed by a slower positive complex, and then a final negative peak, with each complex lasting 1-2 minutes) – see the diagram at right. Together, these serve to protect sleep and suppress response to outside stimuli, as well as to aid in sleep-based memory consolidation and information processing. Because sleepers pass though this stage several times during the night, more time is spent in stage 2 sleep than in any other single stage, and it typically constitutes about 45%-50% of total sleep time for adults (or even more in young adults).
Stage 3 (NREM3 or N3) is also known as deep or delta or slow-wave sleep (SWS), and during this period the sleeper is even less responsive to the outside environment, essentially cut off from the world and unaware of any sounds or other stimuli. Stage 3 sleep occurs in longer periods during the first half of the night, particularly during the first two sleep cycles, and represents around 15%-20% of total adult sleep time. Stage 3 is characterized by delta brain waves with a frequency of around 0.5-4 Hz, along with some sleep spindles, although much fewer than in stage 2. Historically, what is now usually described as stage 3 (following the guidelines of the American Academy of Sleep Medicine) was split into two stages, stage 3 and stage 4, depending on the frequency of delta waves (stage 4 was initially defined as when delta waves exceeded 50% of the total).
As well as neuronal activity, other physical indicators such as brain temperature, breathing rate, heart rate and blood pressure are all at their lowest levels during stage 3 sleep. Dreaming is more common during this stage than in the other non-REM sleep stages, although not as common (nor as vivid and memorable) as during REM sleep. This is also the stage during which parasomnias like night terrors, sleep-walking, sleep-talking and bedwetting occur. Information processing and memory consolidation (particularly of the declarative memory) also takes place during this period, as it also does to some extent during the stage 2 and REM stages. It is much more difficult to wake a person during stage 3 sleep, and if awakened at this stage they will often feel very groggy and may take up to 30 minutes before they attain normal mental performance (known as sleep inertia). Children and young adults tend to have more slow-wave stage 3 sleep than adults, and the elderly may experience little or no stage 3 sleep at all.