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Did early mammals turn to night life to protect their sperm?

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Humans are diurnal — we are active in the day and sleep at night. But diurnalism is by far the exception rather the rule in mammals. About 250-230 million years ago, the mammalian ancestors, called the therapsids, became exclusively nocturnal, and stayed so until the demise of the dinosaurs 66 million years ago.

Source: ScienceDaily.com