Physiological and behavioral rhythms are controlled in mammals by a central circadian clock located in suprachiasmatic nuclei of the hypothalamus (SCN). This master clock has outputs to other organs and tissues crucial to keeping the organism properly synchronized. The SCN clock is synchronized by environmental cues, most importantly is the light-dark cycle (LD). Fearful stimuli (i.e. presence of predators) can also present cyclic variations. The de la Iglesia lab has recently shown that timed fearful stimuli during the night can switch the locomotor activity rhythms of mice to the light phase, overriding their natural nocturnal behavior. Interestingly, while the expression of the so-called “clock genes” (which sustain the circadian rhythms at the molecular level) remains unchanged in the SCN, it displays a complete inversion in the amygdala, the brain region that encodes fear. Currently, we aim to determine the pattern of expression of clock-genes in peripheral organs of mice subjected to cycling fear stimuli. Using qPCR, we will assess RNA expression of the clock genes bmal1, per1, and per2 in the adrenal gland, kidney, and liver to determine whether entrainment of activity by cyclic fear also impacts peripheral clocks at the molecular level. We hypothesize that the pattern of expression the clock genes in the liver and kidneys will be modified in mice subjected to nocturnal fear, due to altered feeding and drinking patterns. However, the adrenal gland is difficult to make predictions about the pattern of expression of the clock genes given the fact that preliminary data from our lab showed that cortisol shows two peaks in mice displaying diurnal activity after nocturnal cyclic fear exposure compared to the single peak displayed by nocturnally active mice. Thus, it is unclear if we will observe an inversion peak, much like the amygdala, or a peak similar to the LD cycle.