Jurgen Aschoff was a German physician, biologist, and behavioral physiologist, and he ran a remarkable experiment in the first half of last century. Aschoff placed men and women, individually, into converted World-War II bunkers that blocked out all daylight. The subjects were placed in these isolation rooms for days at a time, without access to any time-keeping devices.
After a 48-hour adjustment period, Aschoff found that all of the subjects had one strange, miraculous thing in common: they each became biphasic, meaning they slept for about six or seven hours at a time, then had a period of wakefulness, then went back to sleep for another “nap,” this time shorter than the first.
They were falling back into the natural state of biphasic sleep cycles, adhered to by much of the animal kingdom.
via: How to Start Napping…and Why You Should