Peripheral clocks tick independently of their master.

The circadian system of mammals is hierarchically organized. The suprachiasmatic nucleus (SCN) in the hypothalamus is considered the master circadian clock adapting to environmental light-dark cycles and synchronizing subsidiary oscillators in peripheral organs. While being an attractive concept, this has never been convincingly shown in vivo. New findings by Sinturel and colleagues (pp. 329-334) in this issue of now show the requirement of the SCN for temporal orchestration of the periphery in living animals. Surprisingly, this study also found that even in the absence of SCN or extra-SCN clocks, peripheral clocks remain rhythmic, indicating previously controversial circadian oscillator coupling within peripheral tissues.