Which process describes the cyclical nature of biological functions aligned with the day-night cycle?

The cyclical nature of biological functions that align with the day-night cycle is best described by circadian rhythm. This refers to the internal biological processes that repeat roughly every 24 hours, influencing various physiological and behavioral functions in organisms, such as sleep-wake cycles, feeding, hormone release, and body temperature regulation. Circadian rhythms are driven by an internal "biological clock" that responds to external cues, primarily light, helping organisms adapt to the changing conditions of day and night.

The term "chronophysiology" encompasses the study of how biological processes are regulated by time, including circadian rhythms, but it is broader and not exclusively focused on the day-night cycle itself. "Biological clock" refers to the mechanism that generates these rhythms but does not specifically describe the pattern of day-night alignment. "Seasonal adaptation" deals with changes organisms undergo in response to seasonal variations, which is a different concept than the daily cyclical patterns of circadian rhythms. Thus, discussing the regular 24-hour cycles, circadian rhythm is the most precise term for this phenomenon.