Most animals have rest/activity/rhythms that are powered by an endogenous timing mechanism, the circadian system. With aging the rhythm becomes weaker .
Middle aged flies exhibit reduced activity level in comparison with young controls. The free-running period significantly lengthened under constant darkness and the rhythm strength diminishes .
The daily rhythm is brain controlled, but its mechanism is encoded by cell-autonomous circadian clock functioning in nearly every body cell .
The molecular machinery of peripheral circadian clocks appears not to change, but some age-related circadian dysfunction are of hormonal origin. Fibroblast period length, amplitude, and phase are identical in those derived from young and old humans, suggesting that the basic clock properties of peripheral cells do not change during aging. However in the presence of the serum from old donors shortens the period length and advances the phase of cellular circadian rhythms compared to treatment with serum from young individuals, indicating that circulating Factors alter human chronotype. This effect is caused by a thermolabil Factor present in serum of older individuals .
The cytoskeleton changes greatly following a Circadian Rhythm. The liver cells of rodents change their structure during the day, with an average size increase about 50% at the end of the night.  o