In China, teachers allow children to sleep in class for around 20 minutes to help improve their learning.
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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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Bats sleep 18-20 hours per day.
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Studies show that the colder the room you sleep in, the more likely you are to have a bad dream.
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