During REM sleep, a deep inner part of the brain known as the pons blocks signals from the brain to the spinal cord, resulting in a form of paralysis. Sleep scientists theorize that such muscle paralysis is necessary to keep us from physically acting out our dreams.
NREM sleep is generally associated less with vivid dreaming, although studies have shown some form of dreaming activity to be a continuous process during sleep. NREM sleep is often divided into 4 distinct stages that represent different states of body and brain activity while falling asleep and during deep sleep.
Stage 1 sleep is drowsiness. It typically lasts about 5 to 10 minutes. The eyes are closed, but if awakened a person probably would say he or she has not slept. One of the goals of the STS is to help you more easily drift into this first drowsy phase of sleep, which then sets the stage for true sleep.
Stage 2 is deeper than Stage 1 drowsiness and a form of true sleep. The heart rate slows, respiration becomes slower and deeper, and body temperature decreases as it gets ready for deep sleep. However, an external noise or disturbance would easily awaken us from this light form of sleep. Stage 2 sleep may last 30 to 45 minutes.
If at times you feel you are lying in bed but not sleeping, yet somehow the hours seem to go by quickly – you probably are in fact lightly sleeping in Stage 2 or deeper. Keep this in mind as you work the STS – Stage 2 sleep counts as legitimate sleep. It is serving its purpose of helping refresh and rejuvenate you for another day.
Stages 3 and 4 are deep sleep, sometimes called slow-wave sleep, characterized by deep slow breathing and regular continuous rhythms of brain activity. Stage 4 shows deeper patterns than stage 3. We are for the most part cut off from the stimulation of the external world in deep sleep, and least likely to be awakened by a noise or other disturbance at these stages. The deep sleep stages may last around 45 minutes or so.
After stage 4 is reached, stages 3 and 2 repeat, then a period of REM or dreaming sleep is attained. So a typical progression is 1-2-3-4-3-2--REM.
In a typical night, each of these sleep cycles lasts from 90 to 110 minutes on average. The first few cycles of the night typically include longer stages 3 and 4, meaning the deepest sleep, with less dreaming. Later in the night, the amount of REM sleep and vivid dreaming increases in each successive cycle. Typically a person will have four to six of these cycles each night, and the amount of REM dreaming on the final cycle will be around an hour. Why sleep occurs in such cycles is currently unknown.
National Institutes of Health
A typical night for a young, healthy adult, showing 5 cycles of REM and NREM sleep. Light gray areas represent non-rapid eye movement (NREM) sleep.