Figure 1.

A conceptual model of the mammalian circadian system. A circadian pacemaker located in the retinorecipient suprachiasmatic nucleus (SCN) in the hypothalamus generates circadian rest-activity, feeding, body temperature and other rhythms that entrain to environmental light-dark (LD) cycles. The daily feeding rhythm provides time cues that entrain circadian oscillators elsewhere in the brain and in most peripheral organs and tissues. Limiting food access to a single daily meal entrains central and peripheral oscillators, decoupling these from the SCN pacemaker which remains entrained to LD. Some SCN-independent food-entrainable oscillators (FEOs) drive daily rhythms of food anticipatory activity. The location of these FEOs and the signals that entrain them to mealtime, remain to be determined. Peripheral clocks could entrain central FEOs via hormonal pathways involving ghrelin, leptin, corticosterone and insulin, although none of these alone are required for food-anticipatory activity rhythms. Solid arrows indicated interactions for which empirical evidence exists, while dashed arrows indicate possible interactions (coupling among oscillators in different structures). For simplicity, some other possible interactions among and between central and peripheral clocks and meal-related cues are omitted.