Periodic biological fluctuation in an organism corresponding to and in response to periodic environmental change, such as day and night or high and low tide.
The internal mechanism that maintains this rhythm even without the apparent environmental stimulus is a "biological clock." When the rhythm is interrupted, the clock's adjustment is delayed, accounting for such phenomena as jet lag when traveling across time zones. Rhythms may have 24-hour ( circadian rhythm ), monthly, or annual cycles. See also photoperiodism .