Sleep is a physical and mental resting state in which a person becomes relatively inactive and unaware of the environment. In essence, sleep is a partial detachment from the world, where most external stimuli are blocked from the senses. Normal sleep is characterized by a general decrease in body temperature, blood pressure, breathing rate, and most other bodily functions. In contrast, the human brain never decreases inactivity. Studies have shown that the brain is as active during sleep as it is when awake. Throughout an eight-hour sleep cycle, a normal adult alternates between two very different states, non-REM and REM (Rapid Eye Movement) sleep.
What is the Sleep Cycle?
Sleep is characterized by two distinct states, non-REM sleep and REM sleep. Non-REM and REM sleep alternate in 90- to 110-minute cycles. A normal sleep pattern has 4-5 cycles. The diagram and descriptions below illustrate the architecture of sleep. This explains why the cycles of sleep have been compared to going up and down a stairway.
The five-step sleep cycle repeats itself throughout a night of sleeping, much like a roller coaster. As a person drifts off, Stage 1 begins. After a few minutes, the EEG detects sleep. It may take anywhere from 30 – 40 minutes to cycle through Stage 2 through 4. Now, an individual backtracks through Stage 3, then Stage 2, and finally into REM sleep. This occurs 4 - 5 times per night.Non-REM Sleep
Non-REM sleep consists of four stages that range from light dozing to deep sleep. Throughout this state of sleep, muscle activity is still functional, breathing is low, and brain activity is minimal. Approximately 75% of the sleep cycle is spent in non-REM sleep. Simple thought processes may be reported if a person is awakened in any stage of non-REM sleep; however, he or she will not usually recall any specific dream.
Wakefulness - This is a state of awareness in which an individual is conscious of his or her surrounding environment and has the ability to interact with it. In the period before sleep, wakefulness is described as quiet wakefulness, where the individual is resting in a relaxed condition with his or her eyes closed. This stage is characterized by alpha and beta waves activity.Wakefulness is characterized by fast, low voltage EEG activity with both alpha and beta waves present. [Picture courtesy of Dr. Russell Rosenberg]
Stage 1
This stage could be termed the "dozing" stage. Only 5% of non-REM sleep is spent in Stage 1. It is the transitional period of very light sleep. Although the muscles and breathing rate begin to relax, the individual can still be awakened easily. It is during this stage that often, a sleeping individual may experience the sensation of falling and jerk suddenly into wakefulness. This is called a hypnic jerk. After this rush of activity subsides, the body begins to drift into a light slumber.
Stage 1 Sleep is a transition period from wakefulness to sleep. It is characterized by low voltage EEG and slow rolling eye movements. [Picture courtesy of Dr. Russell Rosenberg]Stage 2
This stage is often considered the official onset of consolidated sleep. Approximately 45% of non-REM sleep is spent in Stage 2. Eye movements stop and brain waves become larger. Stage 2 Sleep is graphically defined with two distinct brain wave forms called sleep spindles and K-complexes. A sleep spindle is a pattern of EEG waves, consisting of a burst of 11 to 15 hertz waves that last for .5 to 1.5 seconds. A K-complex is high voltage EEG activity that consists of a sharp downward component followed by a slower upward component and lasts more than .5 second.
Stage 2 Sleep is characterized by a lack of eye movements, sleep spindles and K-complexes. Note the K-complexes and sleep spindles. [Picture courtesy of Dr. Russell Rosenberg]Stage 3
As sleep advances progressively deeper, an individual becomes difficult to arouse. A person spends approximately 12% of non-REM sleep in this stage. Actual slow wave sleep begins as large and slow delta waves intermingle with smaller, faster ones.
This stage of sleep is characterized by 20 to 40% of slow wave (delta) sleep. Note the delta waves. [Picture courtesy of Dr. Russell Rosenberg] Stage 4
Stage 4 is characterized by very deep sleep. Of the roughly 75% of non-REM sleep, approximately 13% is spent in this final stage. A person in one of the two latter stages, either 3 or 4, is harder to wake than a person in Stage 1 or 2. People who wake during deep sleep often feel groggy and disoriented for several minutes. By the time a person shifts into Stage 4, the brain produces elta waves almost exclusively.
Greater than 50% of Stage 4 Sleep is characterized by delta waves. Note that there are few graphical differences between Stage 3 and Stage 4. [Picture courtesy of Dr. Russell Rosenberg]REM Sleep
Most dreaming takes place during REM (Rapid Eye Movement) sleep. Periodic eyelid fluttering, muscle paralysis, and irregular breathing, body temperature, heart rate, and blood pressure distinguish REM from non-REM sleep stages. REM sleep is also called "paradoxical" sleep because brain wave activity is similar to an awakened state. It is during REM sleep that the brain blocks signals to the muscles to remain immobile so dreams will not be acted out. Adults spend about 20 - 25% of their sleep cycle in REM sleep.
The EEG activity during REM sleep shows mixed frequency and low voltage with occasional bursts of "sawtooth" waves. Compare this EEG pattern with the wakefulness pattern. [Picture courtesy of Dr. Russell Rosenberg]
