Essentials: Using Light to Optimize Health

**Andrew Huberman** (0:00)
Welcome to Huberman Lab Essentials, where we revisit past episodes for the most potent and actionable science-based tools for mental health, physical health and performance.
I'm Andrew Huberman, and I'm a professor of neurobiology and ophthalmology at Stanford School of Medicine. Today, we are going to discuss light and the many powerful uses of light to optimize our health. One of the reasons why light has such powerful effects on so many different aspects of our biology is that it can be translated into electrical signals in our brain and body, into hormone signals in our brain and body, and indeed into what we call cascades of biological pathways, meaning light can actually change the genes that the cells of your bodies express. And that is true throughout the lifespan. Light is electromagnetic energy. It can cause reactions in cells of your body. It can cause reactions in fruit, for instance, right? You see a piece of fruit and it's not ripe, but it gets a lot of sunlight and it ripens. That's because the electromagnetic energy of sunlight had an impact on that plant or that tree, or even on the fruit directly. Now, the second thing that you need to understand about the physics of light is that light has many different wavelengths. And the simplest way to conceptualize this is to imagine that cover of that Pink Floyd album where there's a prism, you have a white beam of light going into that prism, and then the prism splits that beam of light into what looks like a rainbow. So you've got your reds, your orange, your greens, your blues, your purples, et cetera. Now, the third bullet point to understand about the physics of light is that different wavelengths of light, because of the way that their wave travels, can penetrate tissues to different depths. Every biological function of light has to do with the absorbance or the reflectance of light or light passing through that particular thing, meaning that particular cell or compartment within a cell. I'd like to make it clear how this works by using the three primary examples of how you take light in your environment and convert it into biological events. We have photoreceptors in the back of our eyes. These photoreceptors come in two major types, the so-called rods and the cones. The rods are very elongated, they look like rods, and the cones look like little triangles. The other place, of course, where light can impact our body is on our surface, on our skin. In the top layer of skin, which is called the epidermis, we have caroteno sites and we have melanocytes. With light exposure, those melanocytes will turn on genetic programs and other biological programs that lead to enhanced pigmentation in the skin, which we call tanning. And the third example I'd like to provide is that of every cell of your body. And what I mean by that is that every cell of your body, meaning a cell that is part of your bone tissue or your bone marrow or heart tissue or liver or spleen, if light can access those cells, it will change the way that those cells function for better or for worse. For many organs within our body that reside deep to our skin, light never arrives at those cells. A really good example of this is the spleen. Light will never land directly on your spleen, but the spleen still responds to light information through indirect pathways. Light arriving on the eyes is absorbed by a particular cell type called the intrinsically photosensitive ganglion cell. It's just a name. You don't need to know the name, but if you want, it's the so-called intrinsically photosensitive ganglion cell, also called the melanopsin cell because it contains an opsin, a photopigment that absorbs short wavelength light that arrives through sunlight. Those cells communicate to particular stations in the brain that in turn connect to your so-called pineal gland, which is this little pea sized gland in the middle of your brain that releases a hormone called melatonin. And the only thing you need to know is that light activates these particular cells, the intrinsically photosensitive melanopsin cells, which in turn shuts down the production of melatonin from the pineal gland. So melatonin is a transducer. It's a communicator of how much light on average is in your physical environment. What this means is for people living in the Northern hemisphere, you're getting more melatonin release in the winter months than you are in the summer months. So you have a calendar system that is based in a hormone and that hormone is using light in order to determine where you are in that journey around the sun. Now, this is beautiful, at least to me it's beautiful because what it means is that the environment around us is converted into a signal that changes the environment within us. That signal is melatonin and melatonin is well known for its role in making us sleepy each night and allowing us to fall asleep. Many of you have probably heard before, I am not a big fan of melatonin supplementation. For a number of reasons, but just as a quick aside, the levels of melatonin that are in most supplements are far too high to really be considered physiological. They are indeed super physiological in most cases. And melatonin can have a number of different effects, not just related to sleep, but that's supplemented melatonin. Here I'm talking about our natural production and release of melatonin according to where we are in the 365 day calendar year. Endogenous melatonin, meaning the melatonin that we make within our bodies naturally, not melatonin that's supplemented, has two general categories of effects. The first set of effects are so-called regulatory effects and the others are protective effects. The regulatory effects are, for instance, that melatonin can positively impact bone mass. Melatonin is also involved in maturation of the gonads during puberty, the ovaries and the testes. Although there, the effects of melatonin tend to be suppressive on maturation of the ovaries and testes, meaning high levels of melatonin tend to reduce testicle volume and reduce certain functions within the testes, including sperm production and testosterone production. And within the ovaries, melatonin can suppress the maturation of eggs, et cetera. Now, I don't want anyone to get scared if you've been taking melatonin. Most of the effects of melatonin on those functions are reversible. But I should point out that one of the reasons why children don't go into puberty until a particular age is that young children tend to have chronically high endogenous melatonin, and that is healthy to keep them out of puberty until it's the right time for puberty to happen. I should also mention that melatonin is a powerful modulator of placental development. So for anyone that's pregnant, if you're considering melatonin supplementation, please, please, please talk to your OBGYN, talk to your other doctor as well. You want to be very, very cautious because of the powerful effects that melatonin can have on the developing fetus and placenta. So when we think about light impacting our biology, the reason I bring up melatonin as the primary example of that is A, because melatonin impacts so many important functions within our brain and body, but also because hormones in general, not always, but in general are responsible for these slow modulatory effects on our biology. And so I'm using this as an example of how light throughout the year is changing the way that the different cells and tissues and organs of your body are working, and that melatonin is the transducer of that signal. So in order to get light information to the pineal and thereby get the proper levels of melatonin, according to the time of year, we should all try and get outside as much as possible during the long days of summer and spring. And in the winter months, it makes sense to spend more time indoors. For those of you that suffer from seasonal affective disorder, which is a seasonal depression, or feel low during the fall and winter months, there are ways to offset this. We did an entire episode on mood and circadian rhythms where we described this. So it does make sense for some people to get more bright light in their eyes early in the morning and throughout the day during the winter months as well. But nonetheless, changes in melatonin, meaning changes in the duration of melatonin release across the year are normal and healthy. So provided that you're not suffering from depression, it's going to be healthy to somewhat modulate your amount of indoor and outdoor time across the year.