Many biological processes are driven by biological clocks that, depending on the frequency they generate, are classified into ultradian, circadian and infradian oscillators. In virtually all light-sensitive organisms from cyanobacteria to humans, a circadian timing system adapts cyclic physiology to geophysical time. Recent evidence suggests that even in mammals circadian oscillators function in a cell-autonomous manner. In yeast, an ultradian oscillator regulates cyclic respiratory activity and global gene expression. Circadian oscillators and the ultradian yeast respiratory clock share at least four properties: they follow limit-cycle kinetics, interweave with cellular metabolism, are temperature-compensated and influence the cell division clock.