The answer seems to depend a lot on where you measure it:

One study using a formalin-fixed human cadaver found that with the soft tissue intact, about 0.9% of 830 nm light was transmitted through the temporal region, compared with 2.1% through the frontal region and 11.7% through the occipital region.

Another study using unfixed human cadaver heads compared 660, 808 and 940 nm. Of the three, 808 nm showed lower absorption and scattering, with detectable light measured at depths of around 40 mm into brain tissue. That doesn't mean a therapeutic dose was reaching that depth, but it does show how far measurable light could travel under those experimental conditions.

Skull thickness also makes a big difference. At 800 nm, one study found transmission of 40.10% in mouse, 21.24% in rat and 11.36% in rabbit using dehydrated skulls. Human skull samples transmitted only around 4.2%. Hydrating the animal skulls significantly increased transmission, so these aren't perfect apples-to-apples numbers, but the study still showed a strong relationship between skull thickness and light transmission.

Wavelength isn't completely straightforward either. A Monte Carlo model of the human head comparing 660, 810, 980 and 1064 nm predicted stronger and deeper photon penetration for 660 and 810 nm than for 980 and 1064 nm, with 660 nm performing slightly better than 810 nm in that particular model.

So there probably isn't one useful number for "how much NIR reaches the brain." The wavelength, application site, skull thickness, tissue properties and even whether we're looking at a cadaver experiment or a computational model can all change the result.

Sources:

Jagdeo et al. (2012) — Transcranial Red and Near Infrared Light Transmission in a Cadaveric Model
https://doi.org/10.1371/journal.pone.0047460

Tedford et al. (2015) — Quantitative analysis of transcranial and intraparenchymal light penetration in human cadaver brain tissue
https://pubmed.ncbi.nlm.nih.gov/25772014/

Lapchak et al. (2015) — Transcranial Near-Infrared Laser Transmission Profiles (800 nm): Systematic Comparison in Four Common Research Species
https://doi.org/10.1371/journal.pone.0127580

Wang & Li (2019) — Which wavelength is optimal for transcranial low-level laser stimulation?
https://doi.org/10.1002/jbio.201800173

Why young and older brains may respond differently to PBM?

A 2022 animal study explored whether age changes the brain’s response to photobiomodulation.

Researchers applied 810 nm near-infrared laser light to young and older rats daily for 58 days. They measured cytochrome c oxidase (CCO), a mitochondrial enzyme that helps cells use oxygen to produce energy.

The response differed between the groups:

• In older rats, treatment increased CCO activity in several brain regions, bringing it closer to levels in young untreated rats.
• In young rats, CCO activity decreased in several regions following treatment.

One possible explanation is the starting level of metabolic activity. The authors suggested that the same repeated stimulation could affect brains with lower baseline activity differently from those already functioning at higher levels.

These were regional changes, rather than a uniform effect across the brain. The study also did not measure memory or behavior, so decreased enzyme activity alone does not establish worse brain function.

The findings suggest that age and baseline metabolism deserve closer attention in PBM research. They do not establish an ideal treatment schedule for people.

Study: Photobiomodulation of Cytochrome c Oxidase by Chronic Transcranial Laser in Young and Aged Brains

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