For most people, the word circadian — which comes from the Latin phrase circa diem, meaning “about a day” — refers to the 24-hour cycle that governs when we sleep and wake up. But circadian rhythms are hardwired into virtually every cell of the body, governed by a master clock in the brain that synchronizes clocks in tissues and organs, including the heart. While these clocks are tuned to daily light-dark cycles, they’re also influenced by behavior: sleep, eating, physical activity, and stress.