Few people know there are multiple types of sleeps, not only the normal night sleep of 7-8 Hours, calles the monophasic sleep cycle.

There are 4 polyphasic sleep cycles:
Urberman Cycle: 20 to 30 minutes naps evert 4 hours
Everyman Cycle: 3 hour nap and three 20-minute naps or 1.5 hour nap with 4-5 20 minute naps, with equal amounts of time in between nap.
Dymaxion Cycle: 30 minutes each 6 hours

Biphasic/Siesta Cycle: Sleeping 4-4.5 hours at night and then taking 90 minute nap around noon.

Source: http://www.highexistence.com/alternate-sleep-cycles/

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