Environmental conditions influence the onset and severity of illness and infection, and may compromise survival. Energetically-challenging conditions during winter may directly induce death through hypothermia, starvation, or shock. The ability to forecast and prepare for the arrival of challenging conditions associated with winter (e.g. low temperatures, decreased food) likely confers survival advantages. Siberian hamsters (Phodopus sungorus) stop reproduction and reduce body mass (25%) during short, winter-like day lengths, resulting in energetic savings. Hamsters also increase circulating glucocorticoids and lymphocytes (e.g. T-cells, NK cells), and exhibit enhanced antigen-specific delayed-type-hypersensitivity (DTH) responses in the skin during short days. We tested the hypothesis that Siberian hamsters use short day lengths to signal the onset of winter to mediate the energetic trade-offs among body mass, reproductive function, and immune function. Long- (LD 16:8) and short-day (LD 8:16) hamsters were either food restricted (25%) or provided ad libitum food for 4 weeks; half of all hamsters in each food condition had voluntary access to a running wheel (RUN), and half remained sedentary (SED). Short-day hamsters enhanced DTH responses compared with long-day hamsters under SED ad lib conditions. Exercise enhanced DTH in long-day hamsters, regardless of food intake. Furthermore, food-restriction did not significantly influence DTH in long-day hamsters. In contrast, food-restriction suppressed DTH in short-day hamsters regardless of activity condition, and exercise modestly enhanced DTH only in short-day hamsters with ad lib access to food. In sum, moderate energetic deficiency suppressed DTH in short, but not long-day hamsters, and suggests that hamsters may have evolved to enhance immune responses during winter in preparation for increased metabolic stressors.