The circadian rhythm is the body’s internal 24-hour biological clock that regulates the timing of a wide range of physiological and behavioral processes, including the sleep-wake cycle, body temperature, hormone secretion, digestion, and immune function. The term comes from the Latin “circa diem,” meaning “around a day,” reflecting the approximately 24-hour cycle on which these rhythms operate. The circadian rhythm is governed by a master biological clock in the brain located in the suprachiasmatic nucleus (SCN), a small region in the hypothalamus. This clock synchronizes with external environmental cues — known as zeitgebers (from the German for “time givers”) — the most powerful of which is light. Exposure to natural daylight in the morning suppresses melatonin, elevates cortisol, and increases alertness, while darkness in the evening triggers melatonin release and prepares the body for sleep. A well-synchronized circadian rhythm promotes regular, high-quality sleep, stable mood, and optimal cognitive performance. Disruptions to the circadian rhythm — from shift work, jet lag, irregular sleep schedules, or excessive artificial light at night — can lead to sleep difficulties, daytime fatigue, mood disturbances, and over time may increase the risk of metabolic, cardiovascular, and mental health conditions. Individual differences in circadian rhythm timing explain why some people are natural morning types (“early birds”) and others are evening types (“night owls”), with most people falling somewhere in between. These chronotypes are partly genetically determined. Maintaining a consistent sleep schedule, getting regular morning light exposure, minimizing artificial light in the evening, and keeping regular meal and exercise times are all effective strategies for supporting a healthy circadian rhythm.