The stages of sleep

Although it's common to think of sleep as a time of “shutting down,” sleep is actually an active physiological process. While metabolism generally slows down during sleep, all major organs and regulatory systems continue to function. In fact, sleep can be categorized into distinct. There are two types of sleep: rapid eye movement (REM) sleep and non-REM (NREM) sleep. Changes in brain activity that take place are measured using an electroencephalogram (EEG).

NREM Sleep

The NREM phase consists of four stages:

Stage 1 is a time of drowsiness or transition from being awake to falling asleep. Brain waves and muscle activity begin to slow down during this stage. People in stage 1 sleep may experience sudden muscle jerks, preceded by a falling sensation.

Stage 2 is a period of light sleep during which eye movement's stop. Brain waves become slower, with occasional bursts of rapid waves called sleep spindles, coupled with spontaneous periods of muscle tone mixed with periods of muscle relaxation. The heart rate slows and body temperature decreases.

Stages 3 and 4 (which together are called slow wave sleep), are characterized by the presence of slow brain waves called delta waves interspersed with smaller, faster waves. Blood pressure falls, breathing slows, and body temperature drops even lower, with the body becoming immobile. Sleep is deeper, with no eye movement and decreased muscle activity, though muscles retain their ability to function. It is most difficult to be awakened during slow wave sleep. People who are awakened during these stages of sleep may feel groggy or disoriented for several minutes after they wake up. It also is during this stage that some children experience bedwetting, night terrors, or sleepwalking.

NREM sleep is characterized by a reduction in physiological activity. As sleep gets deeper, the brain waves as measured by EEG get slower and have greater amplitude, breathing and heart rate slow down, and blood pressure drops.

REM Sleep

REM sleep is an active period of sleep marked by intense brain activity. Brain waves are fast and desynchronized, similar to those in the waking state. Breathing becomes more rapid, irregular, and shallow; eyes move rapidly in various directions and limb muscles become temporarily paralyzed. Heart rate increases and blood pressure rises. This also is the sleep stage in which most dreams occur.