Daily rhythmicity in neuroendocrine functions and sleep-wake cycle is controlled by an endogenous circadian timing system, organized in a network of oscillatory structures. At the top of this circadian multi-oscillatory network, there is a master clock located in the suprachiasmatic nuclei of the hypothalamus. The molecular clockwork involves various clock genes, such as Period (Per).
Light is the most potent synchronizer of the suprachiasmatic clock. By contrast, meal time, as modulated by temporal restricted feeding, has much less synchronizing influence on it, except when animals are no longer exposed to a lightdark cycle. In conditions of constant darkness, a daily restricted feeding can synchronize the circadian rhythms controlled by the suprachiasmatic clock. Moreover, in animals fed with regular food available ad libitum and housed in constant darkness, daily reward cues provided by a palatable diet can also entrain the suprachiasmatic clock. This suggests that activation of reward-related network can, to some extent, feedback to the main circadian clock.
Finally, even when animals are exposed to a lightdark cycle, timed calorie restriction (i.e., when only a hypocaloric diet is given each day) is a synchronizer powerful enough to modify the suprachiasmatic clockwork as well as the phase-shifting effects of light. This feedback of metabolic cues to the main clock involves Per1 and Per2 genes.
Taken together, these data indicate that both metabolic and reward cues can modulate the synchronization of the central clock.