The document discusses biological rhythms and clocks. It defines biological clocks as innate timing mechanisms that control periodic functions like metabolism and sleep cycles. The master clock is located in the brain's suprachiasmatic nucleus and coordinates all biological clocks. Examples of biological rhythms discussed include circadian (daily), lunar & tidal, and circannual (yearly) rhythms. Photoperiodism is the response of organisms to changes in light and dark periods and affects functions like flowering, reproduction, and hormone release in plants and animals. Disruptions to natural biological rhythms can cause disorders like jet lag and shift work sleep problems.