C. Requirements for a biological clock. Egg timers and water clocks are good examples of hourglass timers. They measure a single interval and then need to be reset. These measure time intervals between regular oscillations, such as the sweeps of a pendulum. Anatomy of an oscillation - Period refers to the time between repeating points of the cycle, or in other words, the time it takes to complete one cycle.
Period is symbolized by tau.
Most biological rhythms have a period of ca . 24 hours though they vary from 21-27. Amplitude - intensity of oscillations, or in other words, the difference between the peaks and troughs. Oscillating timers are free running - in other words they don't have to be restarted at each period. And they will continue under constant conditions even though the response will eventually damp out.