The biological behaviours follow cyclic pattern in relation to time. The biological cycles may occur annually (circannual) or daily (circadian).
The factors contributing to circadian periods include both the endogenous rhythmicity and environmental events. Although the biological clock usually follows a 24 period cycle, the free-running clock can be shorter or longer than a 24 hour cycle.
The biological cycle must follow the time-cycle to produce the useful biological effects. The clock setting cue is known as “Zeitgeber.”
Thus, the blanks can be filled with free-running; Zeitgebers.