A<unk>  (), or<unk>  cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., endogenous) and responds to the environment (entrained by the environment). These 24-hour<unk> s are driven by a<unk>  clock, and they have been widely observed in animals, plants, fungi and cyanobacteria. The term<unk>  comes from the Latin circa, meaning "around" (or "approximately"), and diēm, meaning "day". Processes with 24-hour cycles are more generally called diurnal<unk> s; diurnal<unk> s should not be called<unk> s unless they can be confirmed as endogenous, and not environmental. Although<unk> s are endogenous, they are adjusted to the local environment by external cues called zeitgebers (German for "time givers"), which include light, temperature and redox cycles. In clinical settings, an abnormal<unk>  in humans is known as a<unk>  sleep disorder.
Circadian rhythm
Menstrual cycle
Ovarian reserve
Metabolic window
Optokinetic response
Genetics of obesity
Basal electrical rhythm
Cardiac rhythmicity