The term “circadian” refers to any biological process that displays an oscillation of about 24 hours. The word comes from the Latin words “circa” (meaning “about”) and “diem” (meaning “day”). The term is used to describe processes such as the sleep-wake cycle and hormone secretion in animals and humans, as well as other rhythmic processes in plants, fungi, and bacteria.
Circadian rhythms are thought to be regulated by an internal “biological clock” that is influenced by external cues such as light and dark. Disruptions to the normal circadian rhythm, such as those experienced by shift workers or individuals with jet lag, have been linked to a number of health problems, including increased risk of obesity, cardiovascular disease, and depression.