Ultradian Rhythm - Sleep Cycle
As you fall asleep the beta wave activity of your brain dies away. It moves through a brief period of more alpha activity before eventually theta waves and finally delta waves take over.
As already mentioned, sleep occurs in cycles of around 90 minutes during which types of brain activity fluctuate.
5 stages of sleep have been identified based on this brainwave activity.
- Stage 1 Drowsiness The eyes are closed during Stage 1 sleep, but if aroused from it, a person may feel as if he or she has not slept. Stage 1 may last for five to 10 minutes.
- Stage 2 This is a period of light sleep. There are spontaneous periods of muscle activity mixed with periods of muscle relaxation. Muscle movements of this kind can be seen in other stages of sleep as a reaction to auditory stimuli. The heart rate slows, and body temperature decreases. At this point, the body prepares to enter deep sleep.
- Stages 3 and 4 These are deep sleep stages, with Stage 4 being more intense than Stage 3. These stages are known as slow-wave, or delta, sleep.
- Stage 5 REM REM (Rapid Eye Movement). Brainwave activity is similar to Stage 1 but the eyes tend to move erratically beneath eyelids. Intense dreaming occurs during REM sleep as a result of heightened cerebral activity, but the mind paralyses the muscles, possibly to keep the body from acting out the dreams and harming itself. The first period of REM typically lasts 10 minutes, with each recurring REM stage becoming longer, The final stage may last as long as an hour.
Quality sleep by definition must include deep sleep (slow wave sleep). It is deep sleep that the body establishes as soon as it falls asleep. In studies where subjects were deprived of sleep, the moment they were allowed to sleep they went rapidly into a deep sleep and stayed there longer.
The following diagram shows the cycles of sleep for a healthy adult. Notice that as the night progresses, less and less deep sleep is required and REM sleep becomes progressively longer.
