Human hormonal responses, such as changes in melatonin, testosterone, and estrogen, are entrained to the external light-dark cycle and change as day length varies.

“showing that across the calendar year, the amount of testosterone and estrogen that human beings make varies such that in longer days, they tend to make more testosterone and estrogen than in shorter days. And this was correlated with things like desire to seek out romantic partners or have romantic interactions with their existing partners, even aggression, although not violent aggression, but sense of kind of willingness to argue and to get into kind of combative states and overall energy and mood.”

“the point is that our perception of time is both conscious, you know, it's waiting, watching the clock tick down, and there are these slower, what we call oscillatory, meaning up and down repeatedly, slower oscillatory events related to day length that are influencing our hormones like melatonin, testosterone, and estrogen, and therefore our mood, our outlook, and even our behavior.”

“this is a very noteworthy study because it really illustrates that sunlight and day length can impact the melatonin pathway and thereby take the foot off the brake, so to speak, on testosterone, estrogen, and the desire to mate.”

“They observed that testosterone levels were lowest in the winter months and were highest in the months of June, July, August, and September.”