circadian rhythm
nounUK /sɜːˈkeɪdiən ˈrɪðəm/
US /sɜːrˈkeɪdiən ˈrɪðəm/
Your body's natural clock that tells you when to sleep and when to wake up.
My circadian rhythm makes me sleepy every night around nine o'clock.
OTHER MEANINGS
2. The daily cycle of light and dark that living things follow.
Plants open their flowers according to their circadian rhythm.
3. A repeating pattern of activity that happens about once every twenty-four hours.
Scientists studied the circadian rhythm of the mice in the lab.
4. The timing system that controls when animals eat, rest, and are active.
Birds use their circadian rhythm to know when to sing each morning.