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Tony Schwartz in his book "The Way We're Working Isn't Working" is big on naps. It's basically the crux of the book. In it, he mentions a study on laboratory mice. They were deprived of sleeping or even being unconscious until they died.

From this Tony Schwatz deduces we have a daily nap.

Michel Jouvet[1] was the researcher who developed Modafinil. In one of his experiments, Jouvet deprived cats of sleep until they drowned in a vat of water. On average, the cats survived 35 days with only micro-sleep[2].

[1] https://en.wikipedia.org/wiki/Michel_Jouvet [2] http://psychology4a.com/sleep8.htm