What is biphasic sleep and is it healthy for humans?

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In fact, humans are the only mammals who routinely sleep only at night, says Sarah Mednick, Ph.D., professor of cognitive science at the University of California, Irvine, and author of The Hidden Power of the Downstate: The New Science to Ignite Energy, Enhance Sharpness, and Reclaim Balance. “All other animals tend to nap during the day and then get up in the middle of the night,” she says. "The idea that we should be sleeping only during the night and not during the day is more modern, and maybe not as natural to our actual circadian rhythm,” she adds.