Melatonin and immune function: hype or hypothesis?

Outside of the tropics, the combined challenge of food shortage and low temperature makes winter a particularly difficult time for reproduction and survival. Traditionally, vertebrate physiologists interested in seasonal phenomena have focused on reproduction and largely ignored survival. Our goal is to emphasize a new dimension to the study of seasonality by shifting the focus from reproduction to immune function and survival. Our hypothesis is that mechanisms enhancing immune function have evolved that enable individuals to anticipate predictable seasonal challenges that may adversely affect immunity. Stated differently, we visualize seasonal variations in survival as the interaction between two factors: the suppression of immune response due to changing energetic conditions and an endogenous rhythm of enhancement of immune response that is dependent on photoperiod, clocks, and melatonin. In this article, we review evidence of photoperiod influences on seasonal changes in immune function within the context of energy allocation. Further, we also examine mounting evidence that the pattern of melatonin secretion, mediated by photoperiod, directly influences immune function.