Tagesrhythmus

Tagesrhythmus, commonly known as the circadian rhythm, is an endogenous 24-hour cycle that regulates a broad range of physiological and behavioral processes. In humans, the cycle is driven by the internal clock in the suprachiasmatic nucleus of the hypothalamus and synchronized to the environment mainly by light–dark cues, with nonphotic signals such as activity and feeding contributing as well.

Core processes include the sleep–wake cycle, fluctuations in core body temperature, and rhythmic hormone secretion. Melatonin rises in the evening to promote sleep, while cortisol tends to peak in the morning to support wakefulness. Alertness and cognitive performance vary across the day, reflecting the underlying circadian timing along with individual chronotypes.

Entrainment is the alignment of the internal clock to the 24-hour day. Light exposure is the dominant zeitgeber, though social schedules and meals also influence timing. People differ in chronotypes—predispositions toward morning or evening activity—which can shift with age and lifestyle.

Disruptions to the Tagesrhythmus, such as shift work, jet lag, or circadian rhythm disorders (delayed or advanced sleep phase disorders, non-24-hour sleep–wake disorder), can impair sleep quality, metabolism, mood, and performance. Management emphasizes regular sleep schedules, timed light exposure, and behavioral or medical interventions as appropriate.

Researchers study the Tagesrhythmus using markers such as melatonin onset and core body temperature, and by monitoring daily activity with actigraphy. The concept intersects with ultradian and infradian rhythms in the broader field of chronobiology.