Are Humans Seasonally Photoperiodic?

Humans exhibit seasonal variation in a wide variety of behavioral and physiological processes, and numerous investigators have suggested that this might be because we are sensitive to seasonal variation in day length. The evidence supporting this hypothesis is inconsistent. A new hypothesis is offered here—namely, that some humans indeed are seasonally photoresponsive, but others are not, and that individual variation may be the cause of the inconsistencies that have plagued the study of responsiveness to photoperiod in the past. This hypothesis is examined in relation to seasonal changes in the reproductive activity of humans, and it is developed by reviewing and combining five bodies of knowledge: correlations of human birthrates with photoperiod; seasonal changes in the activity of the neuroendocrine pathway that could link photoperiod to gonadal steroid secretion in humans; what is known about photoperiod, latitude, and reproduction of nonhuman primates; documentation of individual variation in photoresponsiveness in rodents and humans; and what is known about the evolutionary ecology of humans.