The Sleep Cycle
According to scientists, sleep is a set of conditions that occur in the brain. Sleep can be thought of as two broad states that can be further divided into four stages. The first state is the deep slow-wave phase which includes deep sleep and insensitivity. The second phase in the Rapid Eye Movement phase, where the brain seems awake and the person experiences dreams. REM sleep was discovered when it was noticed that certain changes always accompanied rapid eye movements, which is when the corneal bulges of the eye move around. These changes included distinctive brain wave patterns, a change in breathing, and other departures from normal sleep patterns. A basic ninety minute sleep pattern also emerged from these observed changes. Each ninety minute cycle progresses from light sleep to deeper sleep, then back to light sleep, ending with a dream (Coren, 1996).
Researchers usually refer to the intensity of sleep in four defined stages of slow wave sleep and REM sleep. When a person falls asleep, they experience stage 1, which is the first of four stages of NREM sleep. NREM sleep is sometimes called "quiet sleep" because it includes slow, regular breathing, slow, regular brain activity, and an absence of body movement. The NREM stage only lasts for a few minutes, while the person is drifting in and out of sleep. Stage 2 is considered to be the first true sleep state, although it is still easy to awaken the person. The next stage, stage 3, lasts about ten minutes, and the person is considered to be sleeping moderately deeply. Stages 3 and 4 are considered to be the deep slow-wave sleep periods. Stage 4 is the stage of deepest sleep and it is very difficult at this point to arouse the sleeper. If one is able to wake the sleeper successfully, the person will probably be disoriented for a few minutes. After stage 4, REM sleep, during which dreams are experienced, occurs (Dement, 1976).
This NREM-REM cycle continues throughout the night, for as long as the person continues to sleep. The more sleep cycles a person gets through, the longer the REM sleep periods become. This means that people experience more REM sleep and dreaming towards the end of the night. Humans spend about thirty percent of their sleep cycle dreaming, about twenty percent of it in deep sleep, and about fifty percent of the time in light sleep (Coren, 1996).
