A hippo sleeping underwater automatically rises to the surface every ~4 minutes and breathes without waking.
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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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The average cat sleeps about 2/3 of the day.
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Sea otters hold hands while sleeping so they don’t drift away from each other.
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In China, teachers allow children to sleep in class for around 20 minutes to help improve their learning.
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"Dysania" is the state of finding it hard getting up out of bed in the morning.
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