A circadian rhythm is a periodic pattern of organism behavior that repeats after approximately one day has passed. For example, animals inactive during the day and active at night are nocturnal, while day-active animals are diurnal.  The daily pattern of circadian rhythms can be studied by experimentally removing the cues that are commonly used to determine day length, such as sunlight, darkness and temperature.  Such cues are called Zeitgebers. The removal of Zeitgebers results in so-called Constant Conditions (CC), continuous light being one example.  If an animal under CC performs a behavior at approximately the same time as an animal that is in its natural environment, the CC animal is said to have a circadian rhythm.  Such a rhythm is considered endogenous (influenced by the internal environment or "memory" of its former natural environment). If an animal in CC does not perform a behavior at approximately the same time as an animal under natural conditions, or if the animal does not change its behavior at all, the CC animal clearly does not have a circadian rhythm.  Such a rhythm is exogenous (influenced by the current external environment, with no "memory" of former natural conditions).