A biological clock is an internal system in organisms that relates behavior to certain rhythms, sometimes called biorhythms. Functions, such as growth, feeding, or reproduction, coincide with certain external events, including day and night, tides and seasons. This innate sense of timing makes some animals feed during the day when food is available and they can see best. These "clocks" seem to be set by environmental conditions, but if organisms are isolated from these conditions, they still function according to the usual rhythm. If conditions change gradually, the organisms adjust their behavior gradually.