INTRODUCTIONFrom bacteria to humans, almost all organisms can adapt the timing of their physiology to the cyclic changes of their environment, thanks to a naturally-selected intrinsic timekeeping system called the circadian clock. 1) The circadian clock enhances the physiological efficiency and survival of an organism by organizing its behavior and body functions. 2,3) During the circadian day, the organism's physiology is given over to catabolic processes, whereas the anabolic functions of growth, repair and consolidation occur at night. To achieve this schedule in mammals, the circadian clock regulates a number of physiological functions, including sleep and wakefulness, food intake, body temperature, cardiovascular and renal activity, hormone production, hepatic metabolism and immune responses. 4,5) Accordingly, disruption of the circadian clock in humans has been linked to profound effects on health, including insomnia, stomach ailments, depression and cancer. 4,5) In most organisms, the molecular mechanisms underlying the establishment and maintenance of biological rhythms comprise interconnected transcription-translation feedback loops in which some clock factors repress their own transcription once they have attained critical levels. 2,3) These oscillators have the property of being endogenous and cellautonomous systems that maintain their rhythm in the absence of external time cues. 6) Both vertebrates and invertebrates have circadian oscillators scattered throughout their bodies. 7,8) In mammals, the circadian system is composed of both central and peripheral oscillators. 7) The mammalian central clock is located in the suprachiasmatic nucleus (SCN) within the anterior hypothalamus of the brain. 9) This central clock acts as a coordinator and provides time signals via both neural and humoral routes that entrain independent peripheral clocks. Dysfunction of the central clock does not inactivate the peripheral clocks but instead causes individual peripheral oscillators to become temporally uncoupled. [10][11][12] To guarantee that an organism's behavior remains tied to the rhythms of its environment, the circadian clock must be able to reset itself in response to environmental cues. 9,13,14) The main environmental stimulus for organisms is light, which is provided in day-night cycles. Mammals have no photoreceptors in peripheral tissues, 15) so that the effect of light on peripheral clocks is indirect. 13) For the mammalian clock, the SCN integrates photic cues from the retina and uses neural and humoral signals to transmit this information to peripheral clocks, synchronizing them. [16][17][18] This communication between the central and peripheral clocks results in the seamless regulation of fundamental physiological functions. 4,8) Interestingly, peripheral clocks can also respond directly to SCN-independent signals such as feeding and temperature change. 19,20) However, the physiological role of SCN-independent responses of peripheral clocks is not yet fully understood. A recent study has reported that ...