Short answer: Near-infrared (NIR) light can penetrate human scalp and skull to a limited degree, which is why technologies such as functional near-infrared spectroscopy (fNIRS) and transcranial photobiomodulation can interact with superficial brain tissue. But this does not show that ordinary sunlight delivers a therapeutically meaningful NIR dose to the brain.
The interesting scientific question is therefore narrower than the original version of this article suggested: sunlight contains NIR wavelengths that overlap with those used in optical brain technologies, but wavelength overlap alone does not establish the same dose, penetration, geometry or biological effect.
What fNIRS actually proves
Functional near-infrared spectroscopy uses near-infrared light to estimate changes in oxygenated and deoxygenated hemoglobin in the cerebral cortex. NIR wavelengths can pass through scalp and skull better than much of the visible spectrum, allowing some photons to travel into superficial cortical tissue and return to detectors placed on the head.
Modern reviews describe fNIRS as a non-invasive method for monitoring cerebral hemodynamics. The signal attributed to brain tissue is dominated by relatively superficial cortex, and extracerebral tissues such as scalp remain an important source of interference.
2025 review of fNIRS principles and applications.
Study of NIR propagation in the adult human head.
Penetration is real, but only a fraction of the light reaches brain tissue
Human-head penetration measurements vary substantially with wavelength, power, source geometry, tissue thickness and measurement method. Reviews have reported transcranial penetration ranging from fractions of a percent to several percent in humans under different experimental conditions.
That variation matters. Saying “NIR penetrates the skull” is true, but incomplete. The biologically relevant question is how much energy reaches the target tissue.
A review of penetration studies reported human scalp-plus-skull transmission values ranging roughly from 0.2% to 10%, depending heavily on the experimental setup. Other work using higher-powered 810- and 980-nm sources found only a small percentage of surface energy reaching a depth of several centimeters.
Review of visible and NIR penetration through head tissues.
Human-tissue penetration experiments with NIR light.
Why fNIRS does not prove that sunlight stimulates the brain
This is the key distinction.
fNIRS uses controlled optical sources placed directly against or very close to the scalp, with known wavelengths, source-detector spacing and signal-processing methods designed to detect tiny changes in reflected light.
Sunlight is a broad-spectrum source arriving from the environment. Even where its spectrum contains the same NIR wavelengths, the delivered irradiance, angle, exposure geometry and effective dose reaching cortical tissue are different.
So the inference:
“fNIRS works with NIR → sunlight contains NIR → sunlight therefore meaningfully photobiomodulates the brain”
does not follow without dosimetry showing that enough solar NIR reaches the relevant brain tissue to trigger a biological effect.
What about transcranial photobiomodulation?
Transcranial photobiomodulation (tPBM) is a different field. It deliberately applies red or near-infrared light to the head using controlled devices, often around 810 nm or similar wavelengths, with specified irradiance and fluence.
A systematic review of human studies found a number of positive cognitive findings, but only about half of the included clinical studies were randomized controlled trials, and protocols varied substantially. The authors concluded that the approach was promising but required better controlled research.
Systematic review of transcranial photobiomodulation and cognition.
More recently, a randomized double-blind placebo-controlled trial in people with mild cognitive impairment due to Alzheimer’s disease reported improvements in cognitive test scores after 12 weeks of an 808-nm home tPBM protocol. That is clinically interesting, but it tests a purpose-built device and a specific treatment schedule — not sunlight exposure.
Randomized trial of home transcranial photobiomodulation in mild cognitive impairment.
The field still has a dose problem
Even within photobiomodulation research, there is disagreement about whether low-power devices deliver enough light through the human scalp and skull to directly affect neurons at meaningful depth.
A 2024 review challenged some commercial claims, emphasizing that animal results do not automatically scale to the much thicker human scalp and skull, and that some low-power LED devices may deliver very little energy to deeper brain tissue.
2024 review of NIR penetration limits and photobiomodulation claims.
This does not prove that tPBM cannot work. Possible effects may depend on superficial cortical exposure, systemic or vascular mechanisms, device power, wavelength, pulsing and anatomical target. It does mean that “NIR reaches the brain” is not enough information by itself.
Does natural sunlight provide enough NIR to affect the brain?
At present, we do not have strong human evidence showing that the NIR component of ordinary sunlight produces a clinically meaningful direct photobiomodulation effect in the brain.
The hypothesis is physically possible in the weak sense that sunlight contains relevant wavelengths and some NIR can penetrate head tissues. What is missing is the crucial bridge: direct measurement of the solar NIR dose reaching human cortex under realistic outdoor conditions, followed by evidence that this dose changes brain function or health outcomes.
Without that bridge, claims about sunlight “charging mitochondria in the brain” or directly improving cognition through cranial NIR remain speculative.
What about hats and hair?
Any material placed between a light source and the scalp can alter transmission. Hair, skin pigmentation, tissue thickness, wavelength, fabric and weave can all matter.
The previous version of this article gave precise percentage ranges for wool, cotton, polyester and nylon hats. We could not identify reliable experimental evidence supporting those specific numbers, so they should not be treated as established measurements.
The defensible statement is simpler: head coverings and hair can reduce or scatter incident NIR, but the magnitude depends on the material and geometry and should be measured rather than guessed.
Sunlight still affects the brain — just through better-established pathways
None of this means sunlight is neurologically irrelevant.
Light reaching the eyes is a major regulator of circadian timing and sleep-wake biology. Outdoor light exposure also changes behavior and is associated with mood and sleep timing. Ultraviolet exposure drives vitamin D synthesis in skin, although UV also carries well-established skin and eye risks.
These pathways should be kept separate from the hypothesis of direct NIR photobiomodulation of brain tissue.
Bottom line
Fact: Near-infrared light can penetrate human scalp and skull to a limited degree, and fNIRS uses this optical window to measure superficial cortical hemodynamics.
Fact: Purpose-built transcranial NIR devices are being studied for photobiomodulation, with some promising human results but substantial uncertainty about optimal dose, penetration and clinical effectiveness.
Not established: That ordinary sunlight delivers enough NIR through the human head to produce a meaningful direct brain effect.
The overlap in wavelength makes the hypothesis interesting. It does not complete the causal chain. The missing experiment is straightforward in principle: measure realistic solar NIR at the scalp, quantify energy reaching cortex, and test whether that dose produces reproducible physiological or cognitive effects.
References
Medical information
This article may contain published medical evidence, clinical context, personal observations, or hypotheses. These are not equivalent levels of evidence. See the Editorial & Medical Review Policy and Medical Disclaimer. This content is educational and does not provide an individual diagnosis or treatment plan.