If you're one of the third of Americans who have some trouble staying asleep through the night, we have good news. Humans haven't always slept through the night in one long doze. Up until the Industrial Revolution, "segmented sleep" was incredibly common.
We've Had First Sleep, But What About Second Sleep?
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Key Facts In This Video
There is no solid scientific consensus on why we sleep. 01:56
Black rhinos sleep for one to two hours laying down, and three to four hours standing up. 04:23
Dolphins engage in unihemispheric sleep, which means that one half of the brain is resting while the other half is alert. 08:01
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