How large ungulates cope with harsh climatic conditions and nutritional shortness during winter is still not fully understood. We found in red deer a previously unknown physiological mechanism of energy conservation - nocturnal hypometabolism associated with peripheral cooling. Predominantly during late winter night and early morning hours subcutaneous temperature, locomotor activity, and metabolic rate, as indicated by heart rate, could decrease substantially, and the more pronounced, the longer such episodes lasted. Daily mean heart rate of the deer varied substantially during the year with a 60 % reduction at the winter nadir compared to the summer peak. Most of this annual change in metabolic rate was to be expected from varying heat increment of food, caused by changes in forage composition and feed uptake. However, independent of such long-term seasonal variation a significant influence of peripheral hypothermia, locomotor activity and ambient temperature on daily mean heart rate was found. Importantly, during the observed episodes of peripheral cooling, heart rate was not maintained at a constant level, as to be expected from classical models of thermoregulation in the thermoneutral zone, but continued to decrease as subcutaneous temperature dropped. This suggests that the circadian minimum of metabolic heat production and set-point of body temperature regulation varied and reached lowest levels during late winter nights in order to reduce the energetic cost of endogenous heat production. Together with accumulating evidence from other species, our results suggest that among endotherms abandoning defence of high body temperature is not restricted to hibernators and daily heterotherms, but is a common physiological response to energetically challenging situations. In this case, classifying species into non-hibernators, hibernators, estivators, and species employing daily torpor has little conceptual value because the differences that lead to this classification may simply result from the duration and extent of hypometabolism and its interaction with body size.