Circadian clock-mediated control of stem cell division and differentiation: beyond night and day

A biological 'circadian' clock conveys diurnal regulation upon nearly all aspects of behavior and physiology to optimize them within the framework of the solar day. From digestion to cardiac function and sleep, both cellular and systemic processes show circadian variations that coincide with diurnal need. However, recent research has shown that this same timekeeping mechanism might have been co-opted to optimize other aspects of development and physiology that have no obvious link to the 24 h day. For example, clocks have been suggested to underlie heterogeneity in stem cell populations, to optimize cycles of cell division during wound healing, and to alter immune progenitor differentiation and migration. Here, I review these circadian mechanisms and propose that they could serve as metronomes for a surprising variety of physiologically and medically important functions that far exceed the daily timekeeping roles for which they probably evolved.