Influence of photoperiod on hormones, behavior, and immune function

Photoperiodism is the ability of plants and animals to measure environmental day length to ascertain time of year. Central to the evolution of photoperiodism in animals is the adaptive distribution of energetically challenging activities across the year to optimize reproductive fitness while balancing the energetic tradeoffs necessary for seasonally-appropriate survival strategies. The ability to accurately predict future events requires endogenous mechanisms to permit physiological anticipation of annual conditions. Day length provides a virtually noise free environmental signal to monitor and accurately predict time of the year. In mammals, melatonin provides the hormonal signal transducing day length. Duration of pineal melatonin is inversely related to day length and its secretion drives enduring changes in many physiological systems, including the HPA, HPG, and brain-gut axes, the autonomic nervous system, and the immune system. Thus, melatonin is the fulcrum mediating redistribution of energetic investment among physiological processes to maximize fitness and survival.

Non-image forming photoreceptors in the retina initiate a signal transduction pathway through the mammalian brain that regulates pineal melatonin synthesis. Pineal melatonin hormonally carries photoperiodic information to distributed systems throughout the body, where it acts both directly and indirectly to regulate endocrine, neuronal, immunological, and behavioral processes. Changes in melatonin duration provide the hormonal fulcrum that regulates seasonal differences in phenotype and allocation of energy differentially among competing processes, such as growth, reproduction, the immune system, thermoregulation, and cellular metabolism.Research highlights► Changes in day length (photoperiod) alter phenotype. ► Melatonin is the hormonal fulcrum of photoperiod. ► Photoperiodic energy redistribution among physiological processes maximizes fitness.