Using Light (Sunlight, Blue Light & Red Light) to Optimize Health

High-Level Overview

In this solo episode, Andrew Huberman, a professor of neurobiology and ophthalmology at Stanford School of Medicine, explores phototherapy: how light of different wavelengths, delivered at particular times and intensities, changes the cells, organs and hormones of the body. He argues this is not pseudoscience, pointing out that the 1903 Nobel Prize went to Niels Finsen for phototherapy. The episode moves from the physics of light and how it enters tissues, to melatonin and seasonal rhythms, to striking recent studies on UVB light and testosterone, estrogen, romantic passion, pain tolerance and immunity. It then turns to the risks of light at night, including during sleep, and to red and near-infrared light for skin, shift work and age-related vision loss, ending with 40 Hz flickering light as a potential brain therapy.

The tone is structured and teacherly, with Huberman promising clarity for non-biologists and depth for experts, and repeatedly pairing mechanisms with protocols and safety warnings. The episode flows from basic physics to biology to specific, actionable tools.

The Discussion Explores:

• The physics of light: energy, wavelength, and why long wavelengths penetrate deeper into tissue
• Direct and indirect ways light becomes biological signals through the eyes, skin and deeper tissues
• Melatonin as the body's seasonal calendar, its effects, and why Huberman is cautious about supplementing it
• A 2021 study showing sunlight on the skin raises testosterone, estrogen and romantic passion in humans
• How UVB light to the eyes and skin reduces pain and boosts immune function
• Timing light for mood, the danger of light at night, and a study on sleeping in dim light
• Red and near-infrared light for skin, for shift workers, and for reversing age-related vision decline
• Flickering 40 Hz light to induce gamma brain rhythms and offer neuroprotection

Deep Dive into Main Ideas

The physics of light and why wavelength matters

Huberman reduces the physics to a few points. Light is electromagnetic energy traveling in waves, and energy changes whatever it lands on, just as sunlight ripens fruit. White light contains many wavelengths, as in the prism on Pink Floyd's album cover, and there are wavelengths we can't see, from ultraviolet to infrared (which pit vipers can sense). Crucially, wavelength determines how deep light penetrates: short-wavelength light like UV, blue and green mostly affects the skin's surface, while long-wavelength red and near-infrared light can pass through skin into deeper tissues, possibly even bone and marrow.

Different wavelengths are also absorbed by specific organelles such as mitochondria or the nucleus. That precision leads him to say that if you imagined a surgical tool to modulate biology, "light would be the sharpest and the most precise of those tools". Everything depends on absorption: pigments determine which wavelengths are absorbed and which bounce off, just as a metal table glares in sunlight while a black one doesn't.

How light becomes biology: eyes, skin and indirect signals

He gives three examples. Rods and cones in the eye absorb light via photopigments; rods are extremely sensitive and work in dim light, while three cone types absorb long, medium and short wavelengths to create color vision. Melanocytes in the skin absorb UV light and trigger tanning. And any cell that light can reach will change its function. For organs light never reaches, like the spleen, signals travel indirectly from the eyes and skin, in a bucket-brigade fashion, to prompt immune responses.

Light works on fast and slow timescales. Moving from darkness into bright light triggers an adrenaline response via the locus coeruleus, much like dropping into cold water. The slow, integrating effects are just as powerful, and the prime example is the circannual rhythm: the body's calendar based on how much light it gets on average across the year.

Melatonin: the body's calendar and a reason for caution

Melanopsin (intrinsically photosensitive) ganglion cells in the eye detect short-wavelength sunlight and signal to the pineal gland to shut down melatonin. In short winter days, melatonin release lasts longer; in long summer days, it's shorter. Melatonin therefore tells the body what time of year it is. Beyond making us sleepy, endogenous melatonin has regulatory effects: it supports bone mass through osteoblasts, suppresses maturation of the testes and ovaries (which is partly why young children, with high melatonin, don't enter puberty early), and strongly affects placental development. It also has protective effects on immunity and is a potent antioxidant, though Huberman stresses that it's the healthy rise and fall of melatonin, not simply more of it, that confers those benefits.

The protocols: get more sunlight in spring and summer, and it's normal and healthy to spend more time indoors in winter unless you have seasonal depression. "Any light provided it's bright enough will shut down melatonin production", so avoid bright overhead lights if you get up at night, keep lights dim, and use dim red or amber light when you need to see. Shift workers should limit light during their sleep period. He is "extremely cautious" about melatonin supplements because most doses are far above physiological levels, the dose is static rather than seasonal, and melatonin is a powerful hormone sold over the counter. Anyone pregnant should talk to their doctor before using it.

Sunlight on the skin, sex hormones and romantic passion

Humans, like other animals, seek mates more in longer days. Huberman describes two parallel pathways. One is a brake: more light means less melatonin, which lifts suppression of testosterone and estrogen. The other is an accelerator, and it works through the skin, because "your skin is actually an endocrine organ". He walks through a 2021 Cell Reports paper titled Skin exposure to UVB light induces a skin-brain-gonad axis and sexual behavior.

In mice, UVB on the skin increased testosterone, estrogen, gonad size and mating behavior. In humans, after a baseline period of covering up, participants spent about 20 to 30 minutes in midday sun two or three times a week, with short sleeves, shorts and no hat or sunglasses, for a total of 10 to 12 sessions over about a month. Blood tests showed significant increases in estradiol, progesterone and testosterone in both men and women, with larger testosterone increases in men from low-UV countries, paralleling how paler skin makes vitamin D more readily. Across the year, testosterone was lowest in winter and highest from June to September. Both sexes reported more romantic passion, sometimes after just one or two sessions; women emphasized physical and sexual passion, men more obsessive thoughts about their partner. Knockout experiments showed the effect depends on p53 activity in skin keratinocytes. His suggested protocol mirrors the study: 20 to 30 minutes of sun on as much skin as is appropriate, two or three times a week, while avoiding sunburn and taking skin cancer risk seriously. He notes sunscreen would in theory block some of the effect but does not recommend abandoning it.

Light, pain relief and immunity

Pain tolerance rises in longer days. One study found that even a single exposure of the skin to UVB increased beta-endorphins, the body's own painkillers. A very recent study in Neuron showed a second pathway through the eyes: melanopsin cells signal through the ventral lateral geniculate nucleus and intergeniculate leaflet to the periaqueductal gray, which releases endogenous opioids. "Nature rarely relies on one mechanism in order to create an important phenomenon", he says. For chronic pain, he suggests the same 20 to 30 minutes of sunlight two or three times a week, if it can be done safely.

He emphasizes that even on cloudy days you get far more light outdoors than indoors, that glasses and contacts are fine, but that sunlight through car windshields or windows doesn't work because "most windows are designed to filter out the UVB light". Blue blockers have a place in the evening but are a disservice during the day. For winter lows, he suggests a SAD lamp or an inexpensive LED drawing panel of about 930 to 1,000 lux, which he keeps on his desk year-round. People who are blind often retain melanopsin cells and can still benefit, but those with retinitis pigmentosa, macular degeneration, glaucoma or high skin cancer risk should consult their ophthalmologist and dermatologist first. UVB light to the eyes also activates the sympathetic nervous system's connection to the spleen, deploying immune cells, which helps explain fewer colds in summer; so in winter, he advises seeking out light deliberately. UVB exposure to the eyes and skin also speeds wound healing and turnover of skin, hair and nail stem cells. He adds that broad illumination of the skin plus the eyes is likely more powerful than beaming light at one spot.

Light timing, mood and the danger of light at night

Citing Dr. Samer Hattar, head of chronobiology at the National Institute of Mental Health, Huberman says getting as much bright light in the eyes and on the skin as safely possible early and throughout the day benefits mood. But a separate pathway from the eye to the perihabenular nucleus does the opposite at night: bright light between about 10 p.m. and 4 a.m. reduces dopamine and can promote depression. People with low mood should be careful from as early as 8 p.m.

Even light you don't consciously notice during sleep matters. A study in the Proceedings of the National Academy of Sciences had young adults sleep in rooms of either under 3 lux or 100 lux. Neither level changed melatonin, yet one night at 100 lux raised nighttime heart rate, lowered heart rate variability, and increased next-morning insulin resistance. He recommends the free Light Meter app to estimate lux in your environment, and making your sleeping room as dark as possible.

Red and near-infrared light for skin and for shift work

Low-level light therapy (LLLT) with red and near-infrared light has good evidence for acne, scarring, wound healing and unwanted pigmentation, including split-face studies. The mechanism: long wavelengths reach the dermis, where sebaceous glands and skin and hair stem cells live, and act on mitochondria to increase ATP and reduce reactive oxygen species, while mild surface damage triggers renewal. Resistance can develop in some acne cases. He points to a 2018 systematic review of LED therapy in dermatology and a review titled Low-level light therapy in skin: stimulating, healing, restoring. He is skeptical of infrared saunas and whole-body red light for skin, noting most infrared saunas don't get hot enough to deliver sauna's known benefits.

For anyone who must be awake at night, whether for shift work, studying or caring for young children, a study titled Red light: a novel, non-pharmacological intervention to promote alertness in shift workers found that dim red light increased alertness without suppressing melatonin or raising cortisol, which at 9 or 10 p.m. is associated with depression. Use it as dim as you can while still doing what you need to do.

Red light to reverse age-related vision loss

Huberman highlights Glen Jeffery's lab at University College London. Rods and cones are the most metabolically demanding cells in the body and accumulate reactive oxygen species with age. In two human studies, people viewed a low-intensity 670-nanometer red light from about a foot away for two to three minutes a day, for two weeks in one study and about 12 weeks in another. People aged 40 to 72, but not younger people, showed about a 22% improvement on the Tritan test of short-wavelength cone function, a large effect for vision research. The treatment also appeared to reduce drusen, cholesterol-like deposits in the eye. The light must be used early in the day, within about three hours of waking, and the researchers found 670 nm red and 790 nm near-infrared light effective and possibly complementary.

Huberman says he has a red light panel and has started using the protocol himself. His safety guidance: most commercial panels are too bright to look at up close (they often come with eye protection), so choose a distance where the light is comfortable, isn't painful and doesn't make you squint, blinking is fine, and consult an optometrist or ophthalmologist first. He notes the brain's photic avoidance pathway produces discomfort and squinting precisely to warn you when light is too bright, and reminds listeners that retinal neurons never regenerate.

Flickering light and the brain

Most brain areas, like the hippocampus, can't be reached directly by light, and drugs and electrical stimulation have drawbacks. Li-Huei Tsai's lab at MIT has shown that light flickering at 40 Hz entrains gamma brain rhythms well beyond visual areas, a method called GENUS (gamma entrainment using sensory stimulation). In the study Gamma entrainment binds higher-order brain regions and offers neuroprotection, it reduced amyloid plaques and phosphorylated tau, markers of Alzheimer's and age-related cognitive decline, while upregulating pathways that support neurons and synapses. Clinical trials are underway, and Huberman thinks it's too early for a home protocol, adding that flickering light can trigger seizures in people prone to epilepsy. He closes by noting he'll cover red light for testosterone and ovarian health in the future because current human data aren't clear yet.

Conclusion

Huberman's core message is that light is one of the most precise and powerful levers on human biology, as long as you match the wavelength, timing and intensity to the goal. Bright sunlight on the eyes and skin early and throughout the day supports mood, sleep timing, immunity, pain tolerance and, according to recent work, testosterone, estrogen and romantic passion, with about 20 to 30 minutes of skin exposure two or three times a week as a practical target. At night, the rule reverses: keep light dim, use red or amber if needed, and sleep in darkness to protect melatonin, dopamine and metabolic health. Red and near-infrared light offer evidence-backed benefits for skin repair, nighttime alertness and, for people over 40, a brief morning protocol that may restore aspects of vision. Throughout, he urges caution: avoid sunburn, never stare at painful light, and consult doctors when eye or skin conditions are involved.

This is a summary of "Using Light (Sunlight, Blue Light & Red Light) to Optimize Health" from Huberman Lab, giving you the GIST so you can decide if you want the REST. Listen to the full episode.