A circadian rhythm is a roughly - 24-hour cycle in the physiological processes of living beings, including plants, animals, fungi and cyanobacteria. The term 'circadian', comes from the Latin circa, 'around', and dies, 'day', meaning literally 'about a day'. The formal study of biological temporal rhythms such as daily, weekly, seasonal, and annual rhythms, is called chronobiology. In a strict sense, circadian rhythms are endogenously generated, although they can be modulated by external cues such as sunlight and temperature. Most of a person's body systems demonstrate circadian variations.The body systems with the most prominent circadian variations are the sleep-wake cycle, the temperature regulation system, and the endocrine system. The malfunctioning of a person's circadian system, or biological clock, causes circadian rhythm disorders. Circadian rhythms and their relation to health outcomes in the globalised hyper world of the 21st century are now understood to be extremely important.