The term circadian comes from the Latin phrase “circa diem,” which means “around a day.”

Circadian rhythms are 24-hour cycles that are part of the body’s internal clock, helping to make sure that the body’s essential functions and processes are optimized at various points during a 24-hour period.

Different systems of the body follow circadian rhythms that are synchronized with a biological clock in the brain. This internal clock is directly influenced by environmental cues, especially light, which is why circadian rhythms are tied to the cycle of day and night and why our circadian clocks reset every day by the sun’s light/dark (sleep/wake) cycle.

By the way, circadian rhythms exist in all types of organisms. For example, they help flowers open and close at the right time and keep nocturnal animals from leaving their shelter during the daytime when they would be exposed to more predators.