Circadian rhythms are physical, mental, and behavioral changes that follow a 24-hour cycle. These natural processes respond primarily to light and dark and affect most living things, including animals, plants, and microbes. It is like your hair that always grow on your head and other body parts. Chronobiology is the study of circadian rhythms. One example of a light-related circadian rhythm is sleeping at night and being awake during the day. Biological rhythms are repetitive biological processes. Some types of biological rhythms have been described as biological clocks.