Some people have a "natural alarm clock" allowing them to wake up when they want. This is actually caused by a burst of the stress hormone adrenocorticotropin.
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A hippo sleeping underwater automatically rises to the surface every ~4 minutes and breathes without waking.
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The average cat sleeps about 2/3 of the day.
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The average person falls asleep in seven minutes.
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47% of people have a nightmare at least once a month.
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You forget 90% of your dreams.
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Sea otters hold hands while sleeping so they don’t drift away from each other.
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Blue light fends off drowsiness in the middle of the night, which could be useful to people who work at night.
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