During recent times a lot of focus has gone into researching sleep mostly due to the number of sleeping disorders that are being diagnosed. According to ancient myths, sleepers lose control of their minds, flirting with death as their souls wonder freely. Early researchers thought sleep was a time of mental inactivity. In fact, however,research now shows that sleep is an active, complex state.
The brain's electrical activity during sleep can be seen on an Electroencephalogram (EEG). EEG recordings show brain waves which vary in height (amplitude) and speed(frequency) as behaviour or mental processes change. The brain waves of an awake, alert person have high frequency and low amplitude. During a night's sleep, brain waves show distinctive and systematic changes in amplitude and frequency. These EEG changes, along with changes in muscle activity and eye movement have been used to describe five stages of sleep: four stages of quiet sleep, followed by rapid eye movement (REM sleep).
STAGES 1-4: Quiet Sleep
Just before going to sleep you are relaxed, with your eyes closed but still awake. During this time, EEG activity includes alpha waves; muscle tone and eye movements are normal. Once you fall asleep, to pass through sleep stages 1 through 4, collectively called quiet sleep because they are characterised by slowed brain waves, deep breathing, a calm heartbeat, and low blood pressure. Because quiet sleep contrasts with REM sleep, it is sometimes called Non-Rem or NREM, sleep.
As you enter stage 1 sleep, your eyes start to roll lazily. Rhythmatic alpha waves give way to irregular waves known as theta waves. Minuted later, stage 2 sleep begins. The EEG shows rapid bursts of waves called sleep spindles, along with occasional K-complexes, waves with high peaks and deep valleys. Stage 3 sleep shows fewer spindles and k-complexes, but it adds delta waves, which are much slower and ;larger in amplitude. When delta waves appear more than half the time, you are in stage 4 sleep, from which it is quite difficult to be roused. If you are awakened during this stage of deep sleep, you tend to be groggy and confused. The journey from stage 1 to stage 4 has taken about twenty minutes.
After thirty to forty-five minutes in stage 4, you begin a special stage in which delta waves become desynchronised, theta waves reappear, and your eyes move rapidly under closed lids. This is called rapid eye movement (REM) sleep, or active sleep. The EEG now resembles that of an awake, alert person, and physiological arousal - heart rate, breathing, blood pressure - also mimics the waking state. Paradoxically, although the EEG and other measures look like those of an awake person, muscle tone falls at the point of near paralysis. Sudden, twitchy spasms appear, especially in the face and hands, as brainstem and spinal neurons control limb muscle movements. Most of a night's dreams occur during REM sleep though NREM dreams are common as well.
A Night's Sleep
Most people pass through the cycle of sleep stages four to six times each night. Each cycle lasts about ninety minutes, but with a somewhat different itinerary. Early in the night most of the time is spent in stages 3 and 4, with only a few minutes in REM. As sleep continue, it is dominated by stage 2 and REM, from which sleepers finally awaken.
Sleep patterns change with age. An average infant sleeps about sixteen hours a day; an average seventy-two year old, only about six hours. The composition of sleep changes with age, too. REM accounts for half of total sleep at birth but less than 25% in young adults. People may vary widely from these averages, however; some people feel well rested after four hours of sleep whereas others of similar age require ten hours to feel satisfied.
For further information or if you are experiencing difficulties sleeping you can view: http://www.medicinenet.com/sleep/article.htm or www.bbc.co.uk/science/humanbody/sleep/