What is circadian rhythm?

• A. The external cues that regulate sleep.
• B. The body's near 24-hour internal clock.
• C. A hormone that signals sleep onset.
• D. The brain's automatic reflex system.

Answer: (B)

Circadian rhythm is the body's near-24-hour timing system that governs daily cycles like wakefulness, sleep, hormone release, and body temperature. The best description is that it is an internal clock operating in roughly a 24-hour cycle, primarily located in the brain’s suprachiasmatic nucleus, which coordinates these processes throughout the body. Light and other environmental cues act to reset or entrain this clock to fit the day-night cycle, helping you stay alert during the day and sleepy at night, but they are signals to adjust the clock, not the rhythm itself. It’s not a hormone by itself, though hormones such as melatonin and cortisol follow the rhythm; and it isn’t a reflex system, but an integrated timing mechanism that underpins how you feel and perform over the day. For pilots, understanding circadian rhythm is important for managing fatigue, especially with jet lag, night flying, or rotating schedules, because misalignment can reduce alertness and safety.