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