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. 2014 Sep 1;6:234. doi: 10.3389/fnagi.2014.00234

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Copyright © 2014 Kent.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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A simplified model of the mammalian circadian system under normal conditions. The light-entrainable oscillator (LEO) is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The LEO is directly entrained by light-dark cycles in the environment through intrinsically photoreceptive retinal ganglion cells, and generates circadian rest-activity rhythms as well as other rhythms that entrain to the environmental light-dark cycles. The LEO-driven activity rhythms then influence the timing of feeding behavior, which entrains the food-entrainable oscillator (FEO). FEO then drives several neuroendocrine, autonomic, and physiological rhythms throughout the brain and peripheral nervous system. The FEO also sends feedback to the LEO to remain coupled. Under certain conditions when the LEO is dysfunctional, or under conditions of constant darkness, the FEO can drive rest-activity rhythms and other rhythms normally entrained by light.