The period of NREM sleep is made up of stages 1-4. Each stage can last from 5 to 15 minutes. A completed cycle of sleep consists of a progression from stages 1-4 before REM sleep is attained, then the cycle starts over again.
Polysomnography (sleep readings) shows a reduction in activity between wakefulness and stage 1 sleep. The eyes are closed during Stage 1 sleep. One can be awakened without difficulty, however, if aroused from this stage of sleep, a person may feel as if he or she has not slept. Stage 1 may last for five to 10 minutes. Many may notice the feeling of falling during this stage of sleep, which may cause a sudden muscle contraction (called hypnic myoclonia).
This is a period of light sleep during which polysomnographic readings show intermittent peaks and valleys, or positive and negative waves. These waves indicate spontaneous periods of muscle tone mixed with periods of muscle relaxation. The heart rate slows and the body temperature decreases. At this point, the body prepares to enter deep sleep.
These are deep sleep stages, with stage 4 being more intense than Stage 3. These stages are known as slow-wave, or delta, sleep. If aroused from sleep during these stages, a person may feel disoriented for a few minutes.
Usually, REM sleep occurs 90 minutes after sleep onset. The first period of REM typically lasts 10 minutes, with each recurring REM stage lengthening, and the final one may last up to an hour. Polysomnograms show brainwave patterns in REM to be similar to that recorded during wakefulness. In people without sleep disorders, heart rate and respiration speed up and become erratic during REM sleep. During this stage the eyes move rapidly in different directions.
Intense dreaming occurs during REM sleep as a result of heightened brain activity, but paralysis occurs simultaneously in the major voluntary muscle groups. REM is a mixture of encephalic (brain) states of excitement and muscular immobility. For this reason, it is sometimes called paradoxical sleep.
The percentage of REM sleep is highest during infancy and early childhood. During adolescence and young adulthood, the percentage of REM sleep declines. Infants can spend up to 50% of their sleep in the REM stage of sleep, whereas adults spend only about 20% in REM.