a | During continuous culture growth, budding yeast exhibit metabolic cycles of ∼4–5 hours, as measured by oxygen consumption (Box 1). These cycles are comprised of three major phases: oxidative (Ox); reductive, building (R/B); and reductive, charging (R/C). Different classes of genes are upregulated during each of these phases5. b | During hibernation, the torpor state (low body temperature) is accompanied by periodic transitions to interbout arousal states (high body temperature). Shown is a plot of body temperature as a function of time in the golden-mantled ground squirrel (interbout arousals occur every ∼11 days). c | In mice, periodic bouts of wakefulness occur during sleep, every ∼2 hours. Shown is a wake-distribution profile for a typical C57BL/6J female mouse51, with periodic wakefulness bouts highlighted by red lines. Grey areas represent 'dark' hours and white areas represent 'light' hours. Part b is courtesy of S. Martin, University of Colorado School of Medicine, Colorado, USA.