Hair has always grown beneath the sun. Long before a laboratory ever pointed a red light at a scalp, hair follicles were developing under the same daily cycle of light and dark that shaped nearly everything else in the body. It is worth asking what that relationship actually did, before it had a name for it.
Most conversations about hair loss run through genetics, hormones, supplements, and the shampoo aisle. All of it matters. But there is a quieter variable that rarely comes up: light. Skin and scalp read the environment constantly, translating daylight into signals that shape circadian rhythm, hormone timing, vitamin D production, immune activity, and the basic metabolism of cells. Hair follicles were never exempt from that conversation. They were simply left out of it.
What a follicle actually does with its day
A hair follicle looks simple from the outside and is almost anything but, underneath. During the growth phase, it is dividing cells, producing keratin, generating pigment, building connective tissue, and maintaining its own small network of blood vessels, all at once. Few tissues in the body work this constantly.
All of that has to be paid for in energy. Inside each follicle, mitochondria produce the ATP that powers the work. When that production slows, hair tends to answer in a specific way: thinner strands, longer stretches in the resting phase, a slow miniaturisation that is the signature of androgenetic alopecia.
The question researchers have started asking is not only how to intervene on the hair itself, but how to support the energy being produced underneath it.
"Healthy hair doesn't begin with the strand you can see. It begins with the energy produced inside the follicle beneath your skin."
The forgotten half of the day
For most of human history, a day had the shape of the sun moving across it. Morning light marked the start of things. Midday light set hormones and alertness. Evening light shifted toward longer, redder wavelengths before the dark allowed the body to recover.
Most people no longer live inside that shape. Something close to ninety percent of a modern day happens indoors, under artificial light that carries only a narrow slice of the red and near infrared wavelengths sunlight provides in full. Nearly half of the sun's energy at ground level sits in the infrared range, wavelengths that, unlike ultraviolet light, move past the surface of the skin and reach deeper tissue.
Sunlight alone will not reverse hair loss. But it points at something the follicle has always known: it evolved inside a world rich in this kind of light. Photobiomodulation is an attempt to recreate a narrow, controlled piece of that world.
How the light is thought to work
Red light therapy, sometimes called photobiomodulation or low-level light therapy, uses wavelengths absorbed by an enzyme inside mitochondria called cytochrome c oxidase. That absorption appears to set off a chain of smaller responses inside the follicle.
Red light does not appear to force hair to grow. It appears to build the conditions in which a follicle can do what it was already trying to do.
What the research actually shows
In the laboratory
A 2021 study on human hair follicles affected by androgenetic alopecia found that a specific red wavelength, 650 nanometres, increased follicle cell proliferation and slowed the shift out of the active growth phase. The same study picked up changes in gene expression tied to metabolism, immune regulation, and cell communication, suggesting the light was doing more than one thing at once.
In clinical trials
Several randomized controlled trials have reported meaningful gains in hair density and count after roughly sixteen weeks of consistent use, in both men and women with androgenetic alopecia, with few reported side effects. The evidence base has grown considerably over the last decade, though individual results still vary.
Alongside other treatments
Increasingly, red light is studied not as a replacement for existing treatments but as a companion to them. Combining it with minoxidil, platelet-rich plasma, or microneedling has shown better outcomes in some research than any one approach alone, likely because light works on the environment around the follicle rather than on hormones directly. Early research is also looking at how this kind of light interacts with the signalling pathways behind tissue regeneration and stem cell activity within the follicle, work that is still unfolding.
Sunlight, vitamin D, and the rest of the picture
Sunlight shapes hair health in other ways too. Exposure to it triggers vitamin D production in the skin, and vitamin D receptors play a real role in how hair follicles cycle through their stages. Low vitamin D has been linked to several forms of hair loss, including alopecia areata and, in some studies, androgenetic alopecia. Sunlight also helps set circadian rhythm, regulates cortisol, and supports nitric oxide release in the skin, each one an indirect contributor to how tissue functions.
None of this is an argument for more sun. Excessive ultraviolet exposure damages the scalp and the hair shaft, raises oxidative stress, and speeds up skin ageing. The point is not more exposure. It is a different kind of exposure, the red and near infrared wavelengths sunlight always carried, delivered without the ultraviolet cost.
What consistency actually means here
One question comes up more than any other: how strong does a device need to be. The research suggests intensity is not the variable that matters most. Consistency is.
Follicles respond to a signal repeated over time, the way a muscle responds to regular use rather than a single hard effort. Most clinical studies report visible change after three to four months of steady use, several sessions a week. Hair grows slowly by nature. The timeline asks for patience more than it asks for power.
When it's worth seeing a dermatologist instead
This isn't a case against dermatology, or against the treatments already shown to work. Hair loss rarely has one cause. Genetics, hormones, nutrition, stress, autoimmune activity, thyroid function, and general metabolic health all shape how a follicle behaves, and photobiomodulation does not override any of that. If thinning is sudden, patchy, or paired with pain, scarring, or other symptoms, that's a conversation for a dermatologist, not a lighting routine. For the slower, more ordinary kind of thinning, light is one reasonable place to start. Not a replacement for what already works. A way of making the environment around the follicle a little more like the one it was built for.
Hair has always grown beneath the sun. Photobiomodulation does not replace that fact so much as translate a narrow part of it into something that fits indoors, on a schedule, at a wavelength a device can control.