Normally, it takes people about 60 to 90 minutes to reach REM sleep, so they wouldn’t enter this stage during a 20 minute nap. People with narcolepsy often enter REM sleep soon after falling asleep. If a person enters REM sleep in two or more of the five naps in eight minutes or less during the study, it’s highly suggestive of narcolepsy.
Julie Flygare, Wide Awake and Dreaming: A Memoir
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