Left on our own—say, by spending months in an underground chamber not exposed to light or other people, as in some experiments—our behavior gradually drifts so that before long we’re asleep in the afternoons and wide-awake at night.4 What prevents such misalignment in the aboveground world are environmental and social signals such as sunrise and alarm clocks. The process by which our internal clocks synch up with external cues so we wake up in time for work or go to sleep at a reasonable hour is called “entrainment.”
