Circadian Rhythm – Time Measured by the Internal Biological Clock
Kacper Nowak
2026-03-24
7 min. read
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In the natural environment, the activity of organisms is not unequivocally assigned to a specific time of day. Humans experience the peak of daily vitality in the morning hours, but there are also animals that lead a nocturnal lifestyle. However, humans are the only beings who consciously change their natural, daytime activity to compulsory functionality at night, which occurs, for example, during night shift work. Such a pattern conflicts with our biological rhythms, leading to changes in sleep patterns and dietary habits among shift workers that can have serious health implications.
Biological rhythms: Definition, underlying mechanisms, and their impact on human physiology
A substantial proportion of physiological and behavioral processes in living organisms exhibit systematic, recurring fluctuations known as biological rhythms. In humans, these intrinsic oscillations are profoundly interconnected with astronomical cycles—Earth’s axial rotation (circadian rhythm), the planet’s annual orbit around the Sun (circannual rhythm), and the lunar phases (circalunar rhythm)—as emphasized by Adamczak-Ratajczak in her 2014 study. While it may appear that human activity is entirely governed by these natural cycles, evidence distinguishes between exogenous rhythms—driven by environmental cues such as sunlight exposure or ambient temperature—and endogenous rhythms, which originate from an internal "molecular clock" primarily situated in the hypothalamus, specifically within the suprachiasmatic nuclei (SCN).
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