Diurnal variation is the study of changes that occur throughout the course of a day, particularly in biological systems. This can include variations in physiological processes, such as hormone levels, body temperature, and metabolism, as well as behavioral patterns and activity levels. Diurnal variations are influenced by the circadian rhythm, which is the body's internal clock that regulates biological processes over a 24-hour cycle. Understanding diurnal variations can provide insights into how organisms adapt to their daily environment and how disruptions to these rhythms can impact health and well-being.