Illustration of hypersomnia with a sleep graph for a normal subject (top graph) and the graph someone suffering from hypersomnia (lower graph). The normal subject is awake during the day and asleep at night, going from a light sleep to a deep sleep several times at the beginning of the night until they reach REM sleep then straight to REM sleep in the second part of the night, never returning to an awakened state between falling sleep and waking. The subject suffering from hypersomnia goes straight to REM sleep several times a day, without going through the intermediate stages, but also several times a night, going through an awakened state between each REM sleep.