Andrew Huberman explains how sunlight, blue light, and red light affect hormones, mood, immunity, and cellular health
Andrew Huberman Lab Essentials episode on using different wavelengths of light to optimize biological function and health.
Summary
Andrew Huberman, professor of neurobiology and ophthalmology at Stanford School of Medicine, presents a comprehensive overview of how light — particularly sunlight, UVB light, and red/near-infrared light — affects human biology at the cellular, hormonal, and systemic levels. He explains how light is converted into biological signals via the eyes and skin, with melatonin serving as the primary hormonal transducer of seasonal light information. Huberman presents research showing that UVB light exposure to the skin increases testosterone and estrogen, enhances pain tolerance through endogenous opioid release, improves immune function via spleen activation, and accelerates hair, skin, and nail regeneration. He also covers findings from Dr. Glenn Jeffery at University College London demonstrating that two to three minutes of 670-nanometer red light early in the day produced a 22% improvement in visual acuity in people aged 40 and older — a result he describes as a reversal of neuronal aging. A key warning is issued against bright light exposure (including but not limited to UVB) between 10 p.m. and 4 a.m., which activates a neural pathway — the eye-to-perihabenular nucleus circuit — that suppresses dopamine and worsens mood.
Key Takeaways
FULL TRANSCRIPT
Light as a Biological Signal: Physics and Mechanisms
Andrew Huberman: 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 body 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. 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.
The second thing you need to understand about the physics of light is that light has many different wavelengths. The simplest way to conceptualize this is to imagine the 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 — your reds, your oranges, your greens, your blues, your purples, etc.
The third 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 a particular cell or compartment within a cell.
How the Body Receives Light: Eyes, Skin, and Cells
Andrew Huberman: There are three primary examples of how you take light in your environment and convert it into biological events. First, 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 and look like rods. The cones look like little triangles.
The second place 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 keratinocytes and we have melanocytes. With light exposure, those melanocytes will turn on genetic programs and other biological programs that lead to enhanced pigmentation of the skin, which we call tanning.
The third example is that of every cell of your body — 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 beneath our skin, light never arrives at those cells directly. 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.
Melatonin: The Hormonal Calendar
Andrew Huberman: Light arriving on the eyes is absorbed by a particular cell type called the 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 the so-called pineal gland, a little pea-sized gland in the middle of your brain that releases a hormone called melatonin. Light activates these particular melanopsin cells, which in turn shuts down the production of melatonin from the pineal gland.
So melatonin is a transducer — a communicator of how much light, on average, is in your physical environment. For people living in the northern hemisphere, you're getting more melatonin release in the winter months than 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.
This is beautiful, at least to me, 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 that I am not a big fan of melatonin supplementation, for a number of reasons. Just as a quick aside, the levels of melatonin 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 — the melatonin that we make within our bodies naturally — has two general categories of effects. The first set are so-called regulatory effects, and the others are protective effects. The regulatory effects include, 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. The effects of melatonin there 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.
I don't want anyone to get scared if they'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 who is pregnant, if you're considering melatonin supplementation, please talk to your OB/GYN and 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.
When we think about light impacting our biology, the reason I bring up melatonin as the primary example is that melatonin impacts so many important functions within our brain and body, but also because hormones in general are responsible for these slow modulatory effects on our biology. I'm using this as an example of how light throughout the year is changing the way that the different cells, 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 to get outside as much as possible during the long days of summer and spring. In the winter months, it makes sense to spend more time indoors. For those of you who suffer from seasonal affective disorder — a seasonal depression — or feel low during the fall and winter months, there are ways to offset this. 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.
Changes in melatonin, meaning changes in the duration of melatonin release across the year, are normal and healthy. 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.
The other thing to understand is the very firmly established fact that light powerfully inhibits melatonin. If you wake up in the middle of the night, go into the bathroom, and flip on very bright overhead fluorescent lights, your melatonin levels — which would ordinarily be quite high in the middle of the night — will immediately plummet to near zero. If you do that every once in a while, it's not going to be a problem. But if you're doing that night after night, you are really disrupting this fundamental signal that occurs every night regardless of season, which is communicating information about where your brain and body should be in time.
UVB Light, Hormones, and Reproductive Biology
Andrew Huberman: In animals such as mice, but also in humans, exposure to light — in particular UV blue light, meaning short wavelengths of light — can trigger increases in testosterone and estrogen and the desire to mate. But it is not the exposure of light to the eyes. It turns out that it is the exposure of your skin to particular wavelengths of light that is triggering increases in the hormones testosterone and estrogen.
I think the results are best understood by simply going through the primary data. I'm going to review a paper published in the journal Cell Reports, entitled "Skin Exposure to UVB Light Induces a Skin-Brain-Gonad Axis and Sexual Behavior." I want to emphasize that this paper focused on mice in order to address specific mechanisms, because in mice you can knock out particular genes to understand mechanism in a controlled way that isn't possible in humans. The study also explored human subjects — both men and women.
The basic finding was that when mice or humans were exposed to UVB — ultraviolet blue light, short-wavelength light of the sort that comes through in sunshine but is also available through various artificial sources — if they received enough exposure of that light to their skin, there were increases in testosterone observed within a very brief period of time, and also increases in the hormone estrogen. The proper ratios of estrogen and testosterone were maintained in both males and females, at least as far as these data indicate. And mice tended to seek out mating more and mate more. There were also increases in gonadal weight — literally increases in testes size and in ovarian size — when mice were exposed to this UVB light past a certain threshold.
They did not look at testes size or ovarian size in the human subjects. However, because they are humans, they did address the psychology of these human beings and address whether or not they had increases in, for instance, aggressiveness or in passionate feelings, and how their perception of other people changed when they were getting a lot of UVB light exposure to the skin.
UVB light exposure also changed various aspects of female biology related to fertility — in particular follicle growth. Follicle and egg maturation are well-known indices of fertility and of course correlate with the menstrual cycle in adult humans, and are related overall to the propensity to become pregnant. UVB light exposure enhanced maturation of the follicle, which meant that more healthy eggs were being produced.
So, in terms of thinking about a protocol to increase testosterone and estrogen, mood, and feelings of passion, the idea is that you would want to get two to three exposures per week, minimum 20 to 30 minutes of sunlight exposure onto as much of your body as you can reasonably expose.
UVB Light and Pain Tolerance
Andrew Huberman: Another set of very impressive effects of UVB light — whether or not it comes from sunlight or from an artificial source — is the effect on our tolerance for pain. It turns out that our tolerance for pain varies across the year, and that pain tolerance is increased in longer-day conditions. This is occurring via UVB exposure to the skin and UVB exposure to the eyes.
I want to describe two studies that really capture the essence of these results. The first is entitled "Skin Exposure to Ultraviolet B Rapidly Activates Systemic Neuroendocrine and Immunosuppressive Responses." Basically, what they observed is that even one exposure to UVB light changed the output of particular hormones and neurochemicals in the body — such as corticotropin hormone and beta-endorphins, which are endogenous opioids — in order to counter pain and act as a psychological soother. What they found was that exposure to UVB light increased the release of these beta-endorphins.
A second study, published in the journal Neuron, is entitled "A Visual Circuit Related to the Periaqueductal Gray Area for the Anti-Nociceptive Effects of Bright Light Treatment." The periaqueductal gray is a region of the midbrain that contains a lot of neurons that can release endogenous opioids — things like beta-endorphins and enkephalins. These are chemicals that your body can manufacture that act as endogenous painkillers and increase your tolerance for pain. They make you feel less pain overall by shutting down some of the neurons that perceive pain. They're not going to block the pain response so that you burn yourself or harm yourself unnecessarily, but they act as a painkiller from the inside.
The key finding of this study is that light landing on the eyes is captured by these melanopsin cells, which absorb that light and translate it into electrical signals that are handed off to areas of the brain to evoke the release of these endogenous opioids that soothe you and lead to less perception of pain.
Practical Protocols for Light Exposure
Andrew Huberman: For those of you thinking about tools and protocols: try to get some UVB exposure, ideally from sunlight. The 20-to-30-minute protocol two or three times per week is an excellent one. Even on a cloud-covered day, you are going to get far more light energy — more photons — through cloud cover than you are going to get from an indoor artificial light source. If you see some sunlight throughout the day, you would do yourself a great favor to get into that sunlight.
Never look at any light — artificial, sunlight, or otherwise — that is so bright that it's painful. It's fine to get that light arriving on your eyes indirectly. It's fine to wear eyeglasses or contact lenses. In fact, those lenses will serve to focus that light onto the very cells that you want those light beams delivered to. Whereas sunglasses that are highly reflective, or trying to get your sunlight exposure through a windshield of a car or through a window, simply won't work. Most windows are designed to filter out UVB light.
And if you're somebody who's really keen on blue blockers and you're wearing them all day — don't wear them outside. You're probably doing yourself a disservice by wearing them in the morning and in the daytime. There certainly is a place for blue blockers in the evening and nighttime if you're having issues with falling and staying asleep. But blue blockers are really blocking those short-wavelength UVB wavelengths of light that you need to arrive at your retina and onto your skin in order to get these powerful biological effects on hormones and on pain reduction.
These data also might make you think a little bit about whether you should wear short sleeves or long sleeves, shorts or pants. If you're completely cloaked in clothing and only your hands, neck, and face are exposed, versus being outside in shorts and a t-shirt, you're going to get very different patterns of biological signaling activation in those two circumstances.
Many of you are probably wondering whether you should seek out UVB exposure throughout the entire year or only in the summer months. That's going to depend on whether you experience depression in the winter months — so-called seasonal affective disorder. Some people have mild forms, some have severe forms, and some people love the fall and winter and the shorter days. It really has to be considered on a case-by-case basis.
I personally believe — and this was reinforced by the director of the chronobiology unit at the National Institute of Mental Health, Samer Hattar — that we would all do well to get more UVB exposure from sunlight throughout the entire year, provided we aren't burning our skin or damaging our eyes. During the winter months, if you do experience a drop in energy or an increase in depression or psychological lows, it can be very beneficial to access a SAD lamp. Or if you don't want to buy a SAD lamp because they can be expensive, you might do well to simply get an LED lighting panel, which is very inexpensive by comparison. I actually have one and I position it on my desk all day long. I also happen to have skylights above my desk. I'm fairly sensitive to the effects of light, so in longer days I feel much better than I do in shorter days. I've never suffered from full-blown seasonal affective disorder, but I keep that light source on throughout the day throughout the year. But I also make it a point to get outside and get sunlight early in the morning and several times throughout the day.
People who are blind, provided they still have eyes, often maintain these melanopsin cells. So even if you're low vision or no vision, getting UVB exposure to your eyes can be very beneficial for mood, hormone pathways, and pain reduction.
A cautionary note: people who have retinitis pigmentosa, macular degeneration, or glaucoma, as well as people who are especially prone to skin cancers, should definitely consult with their ophthalmologist and dermatologist before increasing the total amount of UVB exposure they're getting from any source, sunlight or otherwise.
UVB Light, Immunity, and Cellular Renewal
Andrew Huberman: There are additional very interesting and powerful effects of UVB light on immune function. All the organs of our body are inside our skin, so information about external conditions — the environment we're in — needs to be communicated to the various organs of your body, such as your spleen, which is involved in the creation of molecules and cells that combat infection.
There are studies showing that if we get more UVB exposure from sunlight or from appropriate artificial sources, spleen and immune function are enhanced. The circuit is well established. Your brain actually connects to your spleen. UVB light arriving on the eyes is known to trigger activation of the neurons within the so-called sympathetic nervous system — part of the larger autonomic nervous system, meaning it's below conscious control. It controls your heartbeat, your breathing, and it also activates your immune system.
When we get sufficient UVB light in our eyes, a particular set of connections within the sympathetic nervous system is activated, and our spleen deploys immune cells and molecules that scavenge for and combat infection. The soldiers of your immune system — the chemicals and cell types that combat infection — are in a more ready, deployed stance.
We often think about summer and spring as having fewer infections floating around, but in fact there aren't fewer infections floating around. We are simply better at combating those infections. What this means in terms of a tool: during the winter months, we should be especially conscious of accessing UVB light to enhance our spleen function, to make sure that our sympathetic nervous system is activated to a sufficient level to keep our immune system deploying all those killer T-cells and B-cells and cytokines so that when we encounter infections, as we inevitably will, we can combat them well.
As a related aside: it is well known that wound healing is faster when we are getting sufficient UVB exposure. It is also known that the turnover of hair cells — the stem cells that give rise to hair, which live in little niches in our skin — means that your hair grows faster in longer days. That too is triggered by UVB exposure, not just to the skin but to the eyes. A study published in the Proceedings of the National Academy of Sciences showed that the exposure of those melanopsin ganglion cells in your eyes is absolutely critical for triggering the turnover of stem cells in both the skin and hair, and also it turns out in nails.
So if you've noticed that your skin, hair, and nails look better and grow faster in longer days, that is not a coincidence and not just your perception. Hair grows more. Skin turns over more — meaning it's going to look more youthful, as older skin cells are replaced with new cells. All the renewing cells and tissues of our body are going to proliferate more when we're getting sufficient UVB light to our eyes and also to our skin.
Light at Night: The Dopamine Warning
Andrew Huberman: There is another time — or rather, a time of night — in which UVB can be leveraged to improve mood, but it's actually the inverse of everything we've been talking about up until now.
We have a particular neural circuit that originates with those melanopsin cells in our eye and bypasses all the areas of the brain associated with circadian clocks. So everything related to sleep and wakefulness — but specifically dedicated to the pathways involving the release of molecules like dopamine, serotonin, and some of those endogenous opioids discussed earlier — involves a brain structure called the perihabenular nucleus. The perihabenular nucleus gets input from the cells in the eye that respond to UVB light and frankly to bright light of other wavelengths as well, because if a light is bright enough, even if it's not UV or blue light, it can activate those cells in the eye. Those cells communicate to the perihabenular nucleus. And as it turns out, if this pathway is activated at the wrong time of each 24-hour cycle, mood gets worse. Dopamine output gets worse. Molecules that are there specifically to make us feel good are actually reduced in their output.
Avoiding UVB light at night is a way to prevent activation of this eye-to-perihabenular pathway that can actually turn on depression. To be very direct and succinct: avoid exposure to UVB light from artificial sources between the hours of 10 p.m. and 4 a.m. If you view UVB light during those hours, you activate those neurons in your eye very potently, and if those cells communicate to the perihabenular nucleus — which they do — you will truncate or reduce the amount of dopamine that you release. So if you want to keep your mood elevated, get a lot of UVB light throughout the day. And at night, be very cautious about getting UVB exposure from artificial sources.
I wouldn't want people to become so neurotic about UVB exposure that they won't flip on a light at all. But you would do well, for instance, to put any artificial lights you have on in the evening low in your physical environment, because these melanopsin cells reside in the lower half of our eyes and view the upper visual field — which makes sense, because they were designed to essentially respond to sunlight coming from above us. And try to dim those lights as far down as you safely can.
Red Light, Mitochondria, and Skin Biology
Andrew Huberman: I'd like to shift our attention to the other end of the light spectrum — to talk about red light and infrared light, which are long-wavelength light.
You're probably asking how shining red light on our skin can impact things like acne and wound healing. To understand that, we have to think back to how long-wavelength light such as red light and near-infrared light — which is even longer than red light — can pass through certain surfaces, including our skin. Our skin has an epidermis on the outside and a dermis in the deeper layers. Red light and infrared light can pass down into those deeper layers, where they can change the metabolic function of particular cells.
Let's take acne as an example. Within the dermis, the deep layers of the skin, we have what are called sebaceous glands that make the oil present in our skin. Those sebaceous glands are often nearby hair follicles. The sebaceous gland is where the oil is created that gives rise to acne lesions. Also in the dermis are melanocytes — not just in the epidermis but in the deeper layers. And you have the stem cells that give rise to additional skin cells. If the top layers of the epidermis are damaged, those stem cells can become activated. What happens is the top layers of the skin are essentially burned off by a very low level of burn, and the cells in the deeper layers start to churn out new cells which go and rescue the lesion — essentially clearing it out and replacing it with healthy skin cells. This works in the context of wound healing, getting scars to disappear, and it also works to remove certain patches of pigmentation.
Long-wavelength light can get deep into the skin and can access the so-called organelles within cells — in particular the mitochondria, which are responsible for producing ATP. As cells age, and in particular in very metabolically active cells, they accumulate what are called reactive oxygen species (ROS). As reactive oxygen species go up, ATP energy production in those cells tends to go down. That is a general statement, but a generally true one.
So the way to think about this is that red light passes into the deeper layers of the skin, activates mitochondria, which increases ATP and directly or indirectly reduces these reactive oxygen species. Reactive oxygen species cause cellular damage, cellular death, and for the most part inhibit the way that our cells work. So if you've heard of red light or near-infrared light therapies designed to heal skin, improve skin quality, remove lesions, get rid of scars, or remove unwanted pigmentation — that is not pseudoscience. That is grounded in the very biology of how light interacts with mitochondria and reactive oxygen species.
The key point is that light is activating particular pathways in cells that can either drive death of cells or can make those cells essentially younger by increasing ATP through improved mitochondrial function.
Red Light and Neuronal Regeneration: The Jeffrey Lab Research
Andrew Huberman: In recent years there have been some beautiful examples — not only in the realm of skin biology but in neurobiology — whereby red light and near-infrared light can actually be used to enhance the function of the cells that allow us to see better, and indeed cells that allow us to think better. These are the data from Dr. Glen Jeffery at University College London, a long-standing member of the neuroscience community working on visual neuroscience who, over the last decade or so, has really emphasized the exploration of red light and near-infrared light for restoration of neuronal function as we age.
The Jeffery Lab has published two studies on humans that looked directly at how red light and near-infrared light can improve visual function. They approached these studies with that understanding of how mitochondria, reactive oxygen species, and ATP work. What they did is they had subjects — either younger, in their 20s, or 40 years old or older — view red light of about 670 nanometers. 670 nanometers would appear red to you and me. They had them do that at a distance that was safe for their eyes — about a foot away — and for anywhere from 2 to 3 minutes per day. In one study they had them do that for about 12 weeks, and in the other study for just a couple of weeks.
The major findings were that in individuals 40 years old or older — so in the 40-to-72-year-old bracket — but not in subjects younger than 40, they saw an improvement in visual function. That improvement was an improvement in visual acuity — the ability to resolve fine detail. Using a particular measure of visual function called the Tritan exam, which specifically addresses the function of the so-called short-wavelength cones, the ones that respond to green and blue light, they saw a 22% improvement in visual acuity. In the landscape of visual testing, that is an extremely exciting result.
As we age, we tend to lose rods. We tend to lose other cells within the retina, including the cells that connect the eye to the brain — the so-called ganglion cells. However, because rods and cones are not just among the most metabolically active cells in your entire body but the most metabolically active cells in your entire body, those cells tend to accumulate a lot of reactive oxygen species as we age. Red light of the sort used in these studies was able to reduce the amount of reactive oxygen species in the rods and cones and to rescue the function of this particular cone type — the short-wavelength and medium-wavelength cones.
The important takeaway here is that viewing red light and near-infrared light at a safe distance, for just a couple of minutes each day, allowed a reversal of the aging process of these neurons. So here we're seeing a reversal of the aging process in neurons by shining red light on those neurons.
One of the other things the Jeffery Lab observed was a reduction in so-called drusen. Drusen are little fatty deposits — cholesterol deposits — that accumulate in the eye as we age. Our neural retina, being so metabolically active, requires a lot of blood flow and is heavily vascularized. Drusen are a special form of cholesterol that accumulate in the eye. These red light and near-infrared light therapies were able to actually reduce or reverse some of the accumulation of drusen. So in addition to reducing reactive oxygen species, red light may actually reduce cholesterol deposits in order to improve neuronal function.
Protocols for Red Light Therapy
Andrew Huberman: What should you and I do with these results? First, I want to emphasize that even though these studies are very exciting, they are fairly recent, and more data are always needed.
There are some additional features of these studies that are important to consider. First, the exposure to red light needed to happen early in the day — at least within the first three hours of waking. Nowadays there are a number of different red light panels and different red light sources that fall within the range of red light and near-infrared light that one could use.
If you're somebody who wants to explore red light therapy, here's what you need to do. You need to make sure that the red light source is not so bright that you're damaging your eye. A good rule of thumb is that something isn't painful to look at. Anytime you look at any light source — sunlight or otherwise — that is painful and makes you want to squint or close your eyes, that means it's too bright. Retinal neurons do not regenerate. Once they are gone and dead, they do not come back.
The wavelength of light is important. The authors of this study emphasized that it was red light of 670 nanometers in wavelength and near-infrared light of 790 nanometers in wavelength that were effective, and that those wavelengths could be complementary. A lot of commercially available red light panels combine both red light and near-infrared light. However, most of the panels that are commercially available are going to be too bright to safely look at very close up, and in fact that's why most of those red light panels are designed for illumination of the skin and often arrive in their packaging with eye protectors designed to shield out all the red light.
So take the potential dangers of excessive illumination of the eyes with any wavelength of light seriously. But if you're going to explore 670 and 790 nanometer light for the sake of enhancing neuronal function, set it at a distance that's comfortable to look at and that doesn't force you to squint or make you feel uncomfortable during those two to three minutes of illumination each day.
Red Light at Night for Shift Workers
Andrew Huberman: The studies I just described involve the use of red light early in the day within three hours of waking, for the sake of improving neuronal function. Red light has also been shown to be beneficial late in the day and even in the middle of the night. When I say middle of the night, I'm referring to studies that explored the use of red light for shift workers.
Many people are doing shift work, or they have to work past 10 p.m., or maybe they're taking care of young children in the middle of the night. In that case, red light can actually be very beneficial. Nowadays, there are a lot of sources of red light available, just as red light bulbs — you don't need a panel.
The study I'd like to emphasize in this context is entitled "Red Light: A Novel Non-Pharmacological Intervention to Promote Alertness in Shift Workers." The takeaway is very clear. If you need to be awake late at night for sake of shift work, studying, or taking care of children, red light is going to be your best choice. If the red light is sufficiently dim, it's not going to inhibit melatonin production and it's not going to increase cortisol at night.
Cortisol should be high early in the day — at least elevated relative to other times of day if you are healthy. A late-shifted increase in cortisol — 9 p.m. cortisol, 10 p.m. cortisol — is well known to be associated with depression and other aspects of mental health and mental illness. So if you do need to be awake at night or even all night, red light is going to be the preferred light source. In terms of how bright to make it — as dim as you can while still being able to perform the activities you need to perform. That's going to be your best guide.
Conclusion
Andrew Huberman: Today I covered what I would say is a lot of information. My goal was to give you an understanding of how light can be used to change the activities of cells, organelles within those cells, entire organs, and how that can happen both locally and systemically. Thank you for joining me today for this deep dive into phototherapies — meaning the power of light to modulate our biology and health. And as always, thank you for your interest in science.