Sunlight Exposure

If you had to pick a single environmental input that affects more of your physiology, it would probably be light. The circadian system runs on it. Vitamin D synthesis runs on it. Mood, alertness, reproductive hormones, metabolic timing, even how well you fall asleep tonight all depend on getting the right light at the right time of day.

The modern relationship with sunlight is bad in both directions. We get too little of it during the day (most people spend 90%+ of their time indoors, with light levels that the circadian system barely registers as daytime), and we get too much of the wrong kind at night (when phones, LED lighting, and urban light pollution all signal “still daytime” to a brain that’s trying to wind down).

The popular discourse on sunlight is also a mess. On one side, dermatologists recommend broad-spectrum, chemical-laden, endocrine-disrupting SPF 50 every day of the year, even on overcast winter days indoors. On the other side, wellness influencers who treat the sun as a “cure-all” and suggest the answer to everything is more direct UV exposure to their taints. Both positions misread what the actual research shows.

UV exposure has real benefits and real costs. Skin tone matters for what your dose should look like, and the practical recommendations follow from physiology rather than from whichever camp is loudest.

The clinical territory of vitamin D deficiency, seasonal affective disorder, and skin cancer risk gets touched here but lives more fully in Sleep Disruption & Disorders and across the broader manual. The protocols and timing details are in the Cheat Sheet. 

Why Light Matters

The circadian story from Sleep & Circadian Rhythm Basics makes the basic case: the suprachiasmatic nucleus needs blue-rich daytime light to anchor its rhythm to the actual day-night cycle. Without strong daytime light signals, the SCN drifts. With them, the body’s circadian machinery (peripheral clocks in liver, muscle, gut, immune cells) stays aligned to the master clock, which stays aligned to the sun.

But light does more than entrain the clock.

Vitamin D synthesis: UVB radiation hitting bare skin converts 7-dehydrocholesterol to pre-vitamin D3, which is then converted in the liver and kidneys to the active hormone. The system is the body’s primary route to vitamin D. Dietary sources contribute, but for most people, sunlight does the bulk of the work. Vitamin D status affects bone health, immune function, mood, and much more.

Mood regulation: Light exposure increases serotonin synthesis. Seasonal affective disorder (the cluster of low mood, fatigue, and sleep disturbances common in late autumn and winter) is partly a response to reduced bright-light exposure. Norman Rosenthal at NIH characterized seasonal affective disorder in the 1980s and demonstrated that bright light therapy was an effective treatment. The mechanism appears to involve serotonin and dopamine systems, mediated through the same retinal pathways that entrain the SCN.

Hormonal regulation: Reproductive hormones cycle with daily and seasonal light exposure. Testosterone in men shows daily variation tied to circadian timing and sleep architecture, with morning peaks. Light exposure affects testosterone production over weeks; UV-exposed skin appears to feed back to the hypothalamic-pituitary axis through pathways that are still being mapped.

Cardiovascular effects: UV exposure increases nitric oxide release from skin stores, which produces vasodilation and a small but measurable drop in blood pressure. Richard Weller’s group at Edinburgh has been the main contributor to this research line; the effect is independent of vitamin D and may be one of the reasons the broader epidemiology of sun exposure looks more favorable than the dermatology framing suggests.

Skin barrier function: Moderate UV exposure increases melanin production and thickens the stratum corneum, both of which build tolerance to higher UV doses. This is the mechanism behind “tanning.” Skin that has gradually built up melanin has substantially higher UV tolerance than skin that hasn’t.

Reducing light exposure has broad consequences; the modern indoor lifestyle is paying those costs.

The Light Spectrum and What Each Part Does

Sunlight is a spectrum of electromagnetic radiation across many wavelengths. Different parts of the spectrum interact with the body differently, and most popular discussions oversimplify this dramatically.

Ultraviolet C (UVC, 100–280 nm)

The most energetic UV wavelength and the most biologically dangerous. Almost entirely absorbed by the ozone layer; effectively, no UVC reaches the surface in normal conditions. Used industrially for sterilization (UVC kills microbes effectively), but you don’t encounter it from sunlight. Not a practical concern for daily life unless you’re working with germicidal lamps.

Ultraviolet B (UVB, 280–315 nm)

Partially absorbed by the atmosphere, what reaches the surface is highly dependent on time of day, season, latitude, altitude, and cloud cover. Strongest near solar noon when the sun is high in the sky.

It triggers vitamin D synthesis: When UVB hits skin, it converts 7-dehydrocholesterol in the epidermis to pre-vitamin D3, which spontaneously converts to vitamin D3 (cholecalciferol). This then travels via the bloodstream to the liver, where it’s hydroxylated to 25-hydroxyvitamin D (the form measured in blood tests as “vitamin D level”), and then to the kidneys, where the most metabolically active form, 1,25-dihydroxyvitamin D (calcitriol), is produced. A rough guide to go by is: if the length of your shadow is shorter than you, you will trigger vitamin D synthesis.

It also damages skin DNA: UVB is the primary driver of basal cell carcinoma and squamous cell carcinoma, the two most common types of skin cancer, both highly treatable when caught early. The mechanism is direct: UVB photons damage DNA bonds in skin cells, particularly between adjacent thymine bases (forming “thymine dimers”). Most of this damage is repaired immediately, but accumulated unrepaired damage over decades is what produces skin cancers.

Brief, frequent exposures that don’t cause sunburn appear to produce most of the vitamin D benefit while accumulating much less DNA damage than long, intense exposures that produce burns. Sunburn is the strongest risk factor for melanoma in the literature; gradual, non-burning exposure is associated with much better outcomes. The sun exposure answer is simple: go outside in short bouts to increase UV tolerance without burning, especially after being inside all winter. 

Ultraviolet A (UVA, 315–400 nm)

The longest UV wavelength and the one that penetrates skin deepest. Roughly 95% of the UV reaching the surface is UVA. Less variable across the day than UVB, UVA levels stay relatively high across most daylight hours, including through clouds and glass.

UVA contributes to skin aging through the generation of reactive oxygen species in dermal tissue, breaking down collagen and elastin over decades. It also contributes to skin cancer risk, though less prominently than UVB. The “UV index” reported in weather apps is mostly weighted toward UVB; UVA exposure is harder to track without dedicated equipment.

UVA also has some protective effects worth noting. Richard Weller’s Edinburgh group showed that UVA exposure releases nitric oxide from skin stores, producing measurable cardiovascular benefits. This is part of why the broader epidemiology of sun exposure tends to look better than what dermatology focuses on. The cardiovascular benefits of regular moderate sun exposure may meaningfully offset the cancer risks at the population level.

Visible Light (400–700 nm)

The wavelengths your eyes can see. The blue end of the spectrum (around 480 nm) is what ipRGCs respond to most strongly, which is why blue-rich morning light is so important for circadian entrainment. George Brainard at Thomas Jefferson did the foundational work mapping melatonin suppression to specific wavelengths, with the peak suppression around 460–480 nm.

This is why blue-rich light at night is disruptive. Receiving a daytime signal at night confuses the SCN and delays sleep.

Red and Near-Infrared (620–1100 nm)

These wavelengths penetrate tissue more deeply than visible light and have measurable biological effects through what’s called photobiomodulation. Michael Hamblin at Harvard and Massachusetts General Hospital was the foundational researcher in this space; his decades of work mapped how specific red and near-infrared wavelengths interact with cytochrome c oxidase in mitochondria, increasing cellular energy production and producing measurable therapeutic effects in specific applications.

The legitimate evidence base supports red/near-infrared light therapy for:

• Wound healing acceleration
• Specific musculoskeletal pain conditions
• Some forms of hair regrowth (FDA-cleared devices)
• Age-related macular degeneration (early evidence)

The evidence is weaker or essentially commercial for:

• General “anti-aging” claims
• Most testosterone optimization claims
• Specific mitochondrial restoration protocols
• “Cellular hydration” claims

The most effective wavelengths in the legitimate research are around 630–670 nm (red) and 810–880 nm (near-infrared). Devices marketed at these wavelengths with appropriate power densities have a real research base behind them; many commercial products are at lower-than-effective doses.

A note on exclusion zone (EZ) water claims: these come from Gerald Pollack’s research at the University of Washington, which is genuinely interesting laboratory work on a fourth phase of water that forms near hydrophilic surfaces. The phenomenon is real in his experimental setups. The popular extrapolations that infrared light creates EZ water in your cells, you become dehydrated without enough infrared exposure, and that drinking specific kinds of water restores cellular EZ water content lack substantial research. Treat as speculative; the laboratory phenomenon is real, but the wellness applications are not well-established.

Vitamin D

Michael Holick at Boston University was the dominant researcher in vitamin D physiology for decades. He characterized the synthesis pathway, established the active form (1,25-dihydroxyvitamin D/calcitriol) as a steroid hormone with widespread effects beyond bone health, and made the case that population-level vitamin D deficiency was a major and underappreciated health problem.

Vitamin D receptors are present in essentially every tissue.

The active hormone affects:

• Calcium absorption and bone mineralization (the classical effects)
• Immune cell function, particularly T-cell maturation and antimicrobial peptide production
• Insulin sensitivity and glucose handling
• Mood regulation, possibly through serotonin pathway effects
• Cardiovascular function

Cross-sectional studies have linked low vitamin D status to increased risk of basically every major chronic disease: autoimmune conditions, cardiovascular disease, several cancers, depression, and all-cause mortality.

The Case for Caution About Vitamin D Claims

The challenge: most of those associations come from observational studies, where it’s hard to separate “low vitamin D causes X” from “X causes low vitamin D” from “Y causes both.” Sick people are indoors more. Obesity sequesters vitamin D in adipose tissue and lowers blood levels independent of intake. Aging reduces synthesis efficiency.

The randomized trial evidence, where researchers actually give people vitamin D and look at outcomes, has been substantially less impressive than the observational data predicted. The VITAL trial, a 25,000+ person 5-year RCT published in 2018, found that 2,000 IU/day vitamin D supplementation did not reduce cancer or major cardiovascular events. Multiple other large trials have shown similar null results for outcomes that the observational data suggested vitamin D should improve. However, it has been suggested in the biohacker circles that 2,000 IU is much too small a dose

There’s also the Holick controversy worth surfacing: in 2018, The New York Times published an investigation showing that Holick had received substantial financial support from the indoor tanning industry, the supplement industry, and laboratory companies that profit from vitamin D testing. His clinical recommendations and the cutoffs he proposed for vitamin D deficiency were influential in shifting medical practice toward more testing and supplementation; the financial relationships were not always disclosed. The underlying biology is sound; the specific recommendations require more skeptical reading than the popular wellness discourse usually applies.

What Looks True

• Vitamin D deficiency (defined as 25-hydroxyvitamin D below 20 ng/mL or 50 nmol/L) is real, common, and worth correcting
• Severe deficiency is associated with bone problems and probably modest immune impairment
• Bringing levels into the normal range matters; pushing them into the upper range probably doesn’t add much
• Sunlight is the body’s intended source; supplements work but don’t seem to fully replicate the effects of UVB-induced synthesis
• The “ideal” level is contested; most estimates fall in the 30–50 ng/mL range

Practical Implications

For most light-skinned adults at moderate latitudes, 10-30 minutes of midday sun on bare skin (face, arms, legs) several times a week through spring and summer produces enough vitamin D to last through autumn. Supplementation in winter at 1,000-2,000 IU/day is reasonable for most adults with limited winter sun exposure, taken with vitamin K2 (100–150 mcg) to direct calcium handling appropriately.

Higher latitudes complicate this. Above roughly 35°N or 35°S, UVB from sunlight is insufficient for vitamin D synthesis through much of winter, regardless of how much time you spend outdoors. Auckland (where I live) sits at roughly 37°S, which means winter UVB is genuinely limited and supplementation has more justification than it does at lower latitudes.

Skin Tone and Melanin

This section matters because most sun exposure advice was developed for light-skinned populations and translates badly to people with more melanin in their skin.

Melanin is the body’s built-in UV protection: Skin produces two main types: eumelanin (brown/black, more photoprotective) and pheomelanin (yellow/red, less photoprotective). Skin tone is largely determined by the eumelanin-to-pheomelanin ratio and total melanin content. Higher melanin content provides more protection from UV-induced DNA damage and reduces basal cell, squamous cell, and melanoma risk substantially, though it doesn’t eliminate it.

It also reduces vitamin D synthesis: This is the trade-off: the same melanin that protects against UV damage also blocks UVB from reaching 7-dehydrocholesterol. People with darker skin tones produce vitamin D from sunlight more slowly than people with lighter skin tones. Estimates vary, but a person with very dark skin may need 3-6 times the UV exposure to produce the same amount of vitamin D as a person with very light skin.

In high-latitude countries with substantial populations of African, South Asian, or Middle Eastern descent, vitamin D deficiency rates are dramatically higher in those populations than in the European-descended population due to the latitude-melanin mismatch. Migration patterns over the last few centuries have outpaced the timeline of skin tone adaptation.

Lighter-skinned people in high-UV environments need less exposure to make vitamin D and have less inherent UV protection. The classic dermatology advice (limit exposure, use sunscreen, watch for burns) was largely developed with these populations in mind and is broadly appropriate, though the all-day-everyday framing is too aggressive. Brief midday exposure several times a week is sufficient for vitamin D in most temperate climates.

Darker-skinned people generally need substantially more sun exposure to maintain vitamin D status, particularly at higher latitudes. The risk of UV-related skin cancer is lower (though not zero), and the risk of vitamin D deficiency-related conditions is higher. The “wear sunscreen every day, even indoors” framing is poorly matched to this population’s actual physiology. Aim for longer exposures than the standard recommendations, and consider supplementation actively if you’re at higher latitudes or live a primarily indoor lifestyle.

This is also where seasonality matters more. People with darker skin at high latitudes essentially can’t make vitamin D from sunlight in winter; supplementation becomes more important than for lighter-skinned counterparts.

Sun Avoidance and Sunscreen

The dermatology mainstream over the last several decades has shifted toward aggressive sun avoidance: daily SPF 30+, broad-spectrum coverage, reapplication, and sun-protective clothing. The shift was driven by evidence that severe sunburn correlates with melanoma risk and that cumulative UV damage drives basal and squamous cell carcinomas. The advice is well-intentioned and partially supported.

It’s also probably overcalibrated, and the cost of overcalibration shows up in vitamin D deficiency rates, possibly cardiovascular outcomes, and the deconditioned state most modern adults arrive at when they actually do go outside.

What the Population-Level Data Shows

The clearest challenge to the maximum-avoidance framing comes from Pelle Lindqvist’s Swedish cohort study, which followed 29,518 women for 20 years and tracked their sun exposure habits. Women with the most active sun exposure had lower all-cause mortality than women with the least sun exposure. The protective effect was primarily cardiovascular (sun-exposed women had lower rates of cardiovascular death) and was strong enough to outweigh the higher rates of skin cancer that came with more sun exposure. 

So trade-off matters: total avoidance has costs that aren’t usually counted in the dermatology framing. The Swedish team has explicitly compared aggressive sun avoidance to smoking in terms of population-level mortality risk.

A Reasonable Synthesis

Build melanin gradually: Skin that has gradually built UV tolerance through repeated low-dose exposure handles higher doses with much less damage. Sudden intense exposure on deconditioned skin (the “winter office worker on a tropical beach” scenario) causes most of the dramatic damage. Spring and early summer are the right times to build base tolerance through brief regular exposure, before peak summer UV.

Avoid sunburn: Burns produce DNA damage that overwhelms repair mechanisms. The threshold varies by skin type. What burns one person doesn’t burn another.

Match exposure to skin type: Lighter skin needs less exposure for vitamin D synthesis and tolerates less UV before damage. Darker skin needs more exposure for vitamin D and tolerates more UV.

Use protection for prolonged exposure: Long beach days, hiking at altitude, and other extended high-UV exposures benefit from sun protection. Clothing, hats, shade, and sunscreen for skin that can’t be covered. 

Choose mineral sunscreens where possible: Zinc oxide and titanium dioxide formulations have stronger safety profiles than chemical sunscreens and work through physical UV reflection rather than chemical absorption.

Don’t avoid the sun: Brief daily exposure of unsunscreened skin (face, arms, hands, or legs) when the UV index is moderate to high (the sun is high) is sufficient for vitamin D synthesis without producing sunburn for most skin types. 

Gradual regular exposure that builds melanin appropriately, paired with avoiding burn-level doses and protecting skin during prolonged exposure, fits the actual physiology better than either the maximum-avoidance or the maximum-exposure positions.

Light at Night

Most of the popular discourse on sunlight focuses on how much you’re getting during the day. The other half of the problem is how much artificial light you’re getting at night, and how much it’s interfering with the system.

The mechanism: ipRGCs respond to blue-rich light with melatonin suppression, regardless of when in the 24-hour cycle the light arrives. George Brainard’s dose-response work showed that even relatively modest light intensities at night (100 lux or less) produce measurable melatonin suppression in many people. Bright phone screens at close range can deliver 200+ lux directly to the eye. Standard indoor lighting is often 300-500 lux. Late-evening light at these intensities is functionally telling the SCN it’s still daytime.

The 2011 paper by Joshua Gooley and colleagues at Harvard quantified this directly: exposure to typical room light during the usual hours of sleep suppressed melatonin by more than 50% in healthy adults. Exposure to room light before bed shortened the duration of elevated melatonin during sleep by about 90 minutes.

Delayed sleep onset: Melatonin signals biological night to the rest of the system. Suppressed melatonin delays the system’s transition to sleep mode.

Reduced sleep quality: Even when sleep onset isn’t dramatically delayed, light-disrupted nights tend to produce shallower sleep with less N3 deep sleep.

Disrupted hormonal cycles: Melatonin has receptors throughout the body and influences many systems beyond sleep. Chronic suppression has been associated with metabolic dysregulation and possibly increased cancer risk in shift-worker populations (the IARC classified shift work as probably carcinogenic in 2007, partly on this basis).

Practical Implications for Light at Night

Reduce light intensity in the evening: Dim lights, switch to warmer-spectrum bulbs after sunset, avoid bright overhead lights in the last 1-2 hours before bed.

Reduce blue-spectrum exposure specifically: Blue light filters on phones (Night Shift, f.lux equivalent), blue-blocking glasses, and warm-spectrum lighting all reduce the wavelengths that ipRGCs respond to. The effect of blue blockers reduces melatonin suppression rather than eliminating it.

Use red lighting where you need light: Red-spectrum light (above ~600 nm) doesn’t strongly activate ipRGCs and produces minimal melatonin suppression. Red bulbs in bathrooms, hallways, and bedrooms allow nighttime navigation without circadian disruption. Also, place the lights lower on your visual horizon, if possible, such as placing a small lamp on an end table. 

Sleep in genuine darkness: Even ambient light during sleep can affect circadian alignment. Blackout curtains, eye masks, and removing bright LED indicators from electronics all help.

Don’t overdo the protocols: The wellness industry has produced a lot of expensive solutions (specialized blue-blocking glasses, exotic lighting setups, red-light bulbs from premium brands). The basic moves are to dim the lights, put down the phone an hour before bed, and sleep in a dark room

A note on Andrew Huberman’s content: he’s been one of the loudest voices popularizing the morning sunlight/evening darkness frame, and his framing is broadly correct on this topic. Where his specific protocols (look at the morning sun for X minutes within Y minutes of waking, etc.) are precise, the underlying research is less precise. Treat the general principle as solid and the specific numbers as rough guidance rather than precise prescriptions.