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. All of our mammal ancestors lived in the dark for about 200 years, and the majority still do to this day. Humans are, essentially, nocturnal animals that have reverted back to living in the sun.
