Most people are at least somewhat familiar with the notion of the circadian rhythm, an approximately 24-hour cycle of biological processes in human bodies. (The word “circadian” comes from Latin circa dies meaning “about a day.”) Practically every organism on Earth follows such a cycle, although not always in the same way; for example, many humans work during the day and sleep at night, whereas nocturnal animals sleep during the day and forage at night. The principle remains: Activities of all animals follow such cycles and have been doing so for hundreds of thousands of years.

The human body falls into this cycle by following external cues, called “zeitgebers” (from German zeit meaning “time” and geben meaning “to give”); the main zeitgeber is light. Most animals evolved being exposed to different light conditions in the morning, during the day, and at night.