Sleep Architecture
A polysomnography (PSG) is a test that charts brain waves. These brain activities vary with the sleep-wake cycle. Sleep is a dynamic state with its own architecture. In healthy adults, there are different stages; light sleep (sleep stages 1 & 2: 50%), deep sleep (sleep stage 3: 25%) and REM sleep (25%). It also has a very specific polarity, i.e. the preponderance of deep sleep in the beginning of the night and REM sleep at the end of the night. A sleep cycle corresponds to the succession of stages 1 through 3, followed by REM sleep. This cycle will repeat five to six times during the night, every 90 minutes.
Here a graphical representation a architecture of sleep (hypnogram) of a normal person in good health
To learn about each stage of sleep, click on the color bars on the graph
Or, proceed step-by-step, starting with wake
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Rapid Eye Movement Sleep (REM Sleep)
Several brain waves can be recorded during REM sleep, including theta waves. Theta waves are associated with slow activities, characterized by light sleep and the basic activities of REM sleep. Brain activity close to that of wakefulness is also present and includes alpha and beta waves. Accordingly, REM sleep is mixed brain activities, quite similar to those observed during waking hours. REM sleep is characterized by rapid eye movements, as well as muscle atony. It is the stage of vivid dreams. Although it is difficult to wake somebody from this stage of sleep, this is when certain types of parasomnia can occur, such as nightmares or REM sleep behaviour disorder.
Deep Sleep
(Non-REM Sleep, stage 3)
Both theta and delta waves can be observed during this stage of sleep. Delta waves are associated with the slowest activities, indicating deep sleep. Breathing and pulse become increasingly regular and slow. Muscles are relaxed and the body temperature is even lower. Although it is difficult to wake up a person during this stage of sleep, this is when certain types of parasomnia occur, including sleepwalking, confusional arousals and night terrors. Non-REM sleep is generally followed by REM sleep, and so the cycle continues.
The American Academy of Sleep Medicine (AASM).
Light Sleep (Non-REM Sleep, stages 1,2)
Light sleep comprises stage 1 (the onset of sleep) and stage 2 (the first steps of sleep). The waves charted during this stage are mostly theta waves, like those present during REM sleep. Sleep spindles (bursts of alpha brain activity) and K-complexes (brief slow delta waves) can also be observed. Gentle eye-rolling accompanies slowed breathing and a decreased heart rate. Muscles relax, becoming motionless, and the body temperature is low. A person in this stage of sleep is easily awakened.
Wakefulness
When awake, the brain emits beta or alpha waves. Beta is associated with the fastest activities, indicating overall activation of the cortex. This corresponds to wakefulness with stimulation (i.e. working on the computer). Alpha is also associated with fast activities, but indicate rather a wakeful state of relaxation (i.e. stretched out on a park bench).
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Référence:
American Academy of Sleep Medicine. International Classification of Sleep Disorders: Diagnostic and Coding Manual. 2e éd. Rochester, 2005.
