TIM, PER, tim, and per appear to be working hand in hand in the function of circadian rhythms. Ultradian rhythms are those which cycle in less than 24 hours. Daydreaming, urination, and hunger are all examples of ultradian rhythms. Activity cycles are positively correlated to the size of the organism. Zeitgeber is the term for a cue that resets the circadian clock.
Light is a very common cue used by circadian pacemakers to keep time.
Zeitgber is also known as an environmental cue, and an entraining agent. Conclusion Rhythms are intrinsic to possibly all life forms and occur at all levels of the life form. The digestive tract may be controlled by a rhythm, as is a single cell, and as does a part of a cell such as a nucleus. One organism will have multiply rhythms operating at a given time. The key to understanding any area is knowing the key terms and jargon associated with the topic.