Why do individuals have different chronotypes? To quote Shakespeare, “Some must watch while some must sleep.” From an evolutionary standpoint, consider what life was like for our ancestors ten or twenty thousand years ago. Sleep was necessary for survival, and yet it was a time when we were especially vulnerable to attack by animal predators and violent humans, as well as the occasional hurricane or erupting volcano. The sentinel hypothesis is that when living in groups, animals share the task of nighttime vigilance, some watching over those who sleep.
