Did early mammals turn to night life to protect their sperm?
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.
Paleontology News — ScienceDaily