What are the stages of human sleep?
Normal adult sleep is organized into rapid eye movement (REM) sleep and non–rapid eye movement (NREM) sleep, which is further divided into stages N1, N2, and N3.
This final NREM stage is also called slow-wave sleep (SWS). In classic teaching, NREM was organized into four stages, but in 2007, stages N3 and N4 were combined. Each of these stages has specific electroencephalography (EEG) characteristics.
Typically, adults enter sleep through stage N1. It usually takes 90 to 100 minutes for the first NREM sleep cycle to finish, but once completed, it heralds the first REM period. The hallmark of REM sleep is rapid movement of the eyes compared with the slow eye movements (SEM) seen on electrooculography in stage N1 sleep.
Also defining REM is muscle atonia, which manifests as absence of chin muscle movement and attendant low electromyography tone. The only somatic muscles working in REM are the extraocular muscles and the diaphragm! Curiously, we will see that REM predominant OSA, which is OSA predominant in REM, is associated with occult hyperglycemia.