A circadian rhythm, popularly referred to as body clock, is an endogenously driven (non-reliant on environmental cues), roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria (see bacterial circadian rhythms). The term circadian comes from the Latin circa, meaning "around", and diem or dies, meaning "day". The formal study of biological temporal rhythms such as daily, tidal, weekly, seasonal, and annual rhythms is called chronobiology. Although circadian rhythms are endogenous ("built-in", self-sustained), they are adjusted (entrained) to the environment by external cues called zeitgebers, the primary one of which is daylight.