Sleeping Animals

• Sleep stages
In mammals and birds, sleep is divided into two broad types: rapid eye movement (REM) and non-rapid eye movement (NREM or non-REM) sleep. Each type has a distinct set of associated physiological, neurological, and psychological features. The American Academy of Sleep Medicine (AASM) further divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or slow-wave sleep (SWS).
Sleep proceeds in cycles of REM and NREM, the order normally being N1 → N2 → N3 → N2 → REM. There is a greater amount of deep sleep (stage N3) earlier in the sleep cycle, while the proportion of REM sleep increases later in the sleep cycle and just before natural awakening.
The stages of sleep were first described in 1937 by Alfred Lee Loomis and his coworkers, who separated the different electroencephalography (EEG) features of sleep into five levels (A to E), which represented the spectrum from wakefulness to deep sleep. In 1953, REM sleep was discovered as distinct, and thus William Dement and Nathaniel Kleitman reclassified sleep into four NREM stages and REM. The staging criteria were standardized in 1968 by Allan Rechtschaffen and Anthony Kales in the "R&K sleep scoring manual." In the R&K standard, NREM sleep was divided into four stages, with slow-wave sleep comprising stages 3 and 4. In stage 3, delta waves made up less than 50% of the total wave patterns, while they made up more than 50% in stage 4. Furthermore, REM sleep was sometimes referred to as stage 5.