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How Our Ancestors Used to Sleep Twice a Night - http://disinfo.com/2013/08/how-our-ancestors-used-to-sleep-tw...
"His research found that we didn’t always sleep in one eight hour chunk. We used to sleep in two shorter periods, over a longer range of night. This range was about 12 hours long, and began with a sleep of three to four hours, wakefulness of two to three hours, then sleep again until morning." ‎- nm