Andrew Huberman· PhD
It's been recognized for a very long time, and in fact, there are now data to support the fact that animals of all kinds, including humans, will seek out mates and engage in mating behavior more frequently during the long days of spring and summer. That's right, in seasonally-breeding animals, of course, this is the case, but in humans as well, there is more seeking out of mates and mating behavior in longer day times of year. Now, you could imagine at least two mechanisms by which this occurs. The first mechanism we could easily map to melatonin and the fact that melatonin is suppressive to various aspects of the so-called gonadal axis, which is basically a fancy way of saying that melatonin inhibits testosterone and estrogen output from the testes and from the ovaries. I just want to remind people that both males and females make testosterone and estrogen, although in different ratios, typically, in males versus females, and that both testosterone and estrogen are critical for the desire to mate and for mating behavior. There's a broad misconception that testosterone is involved in mating behavior and estrogen's involved in other behaviors, but having enough estrogen is critical for both males and females in order to maintain the desire to mate, and indeed the ability to mate. I discuss this on the episode on optimizing testosterone and estrogen. So if you'd like more details on that, please see that episode of the Huberman Lab Podcast. Okay, so if melatonin is suppressive to the so-called gonadal axis and reduces overall levels of testosterone and estrogen in males and females and a light inhibits melatonin, then when there's more light, then there's less melatonin and more hormone output from the gonads. And indeed that's how the system works, but that's not the entire story. It turns out that there is a second so-called parallel pathway, meaning a different biological pathway that operates in parallel to the light suppression of melatonin pathway that provides a basis for longer days, inspiring more desire to mate and more mating behavior. So if we think of the first pathway involving melatonin as sort of a break on these reproductive hormones, the second mechanism is more like an accelerator on those hormones. And yet it still involve light. As I'm about to tell you, in animals such as mice, but also in humans, exposure to light, in particular UV blue light, so short wavelengths of light, can trigger increases in testosterone and estrogen and the desire to mate. Now what's especially important about this accelerator on the desire to mate and mating behavior and hormones is that it is driven by exposure to light, but it is not the exposure of light to the eyes. It turns out that it is the exposure