Circadian (from the Latin circa, meaning “about,” and dies, meaning “day”) are the regular, internal body rhythms. Although our lives revolve around a 24-hour day, researchers have found that normal circadian rhythms are more on a 25-hour cycle. Many physiological processes, including the sleep/wake cycle, body temperature, gastric secretion, and kidney function, follow a set pattern. For example, body temperature peaks in the late afternoon/early evening and is lowest between 2:00 A.M. and 5:00 A.M. Blood pressure, heartbeat, and respiration follow rhythmical cycles. The production of urine drops at night, allowing for uninterrupted rest.

Circadian rhythm disturbances occur when sleep/wake cycles are interrupted. They often affect shift workers whose biological clocks are disrupted by conflicting sleep and work schedules. “Jet lag” is another form of circadian rhythm disturbance.