The 2018 Harvard study that changed at-home hair regrowth forever.

In 2018, researchers affiliated with Harvard Medical School's Department of Dermatology published findings on photobiomodulation for androgenetic alopecia — the clinical term for male and female pattern hair loss — that quietly reshaped the entire hair-regrowth industry. The study wasn't particularly large by pharmaceutical standards, and it didn't make front-page news. But it moved a treatment that had lived on the fringe of clinical dermatology for decades into the mainstream, and it opened the door for at-home devices that had, until then, been dismissed as gimmicks.

128 adult participants — men and women with clinically diagnosed androgenetic alopecia — were randomised to either an active 655nm red light therapy device or a visually identical sham device emitting no therapeutic output. Both groups used their device for 25-minute sessions, every other day, for 26 weeks. Neither the participants nor the dermatologists taking trichoscopic measurements knew which arm each subject was on.

The result: the active group showed a 35% increase in terminal hair density versus baseline. The sham group showed statistically insignificant change. Independent imaging by two blinded assessors confirmed the count. The effect was large enough to be clinically meaningful — not just statistically noticeable.

Hair follicles are among the most metabolically demanding structures in the human body. Each follicle cycles through growth, regression, and rest phases, and each phase depends on a steady supply of ATP — the molecule cells use as energy currency. In androgenetic alopecia, follicles progressively starve: growth phases shorten, resting phases lengthen, and the hair produced gets finer and finer until the follicle miniaturises entirely.

Red light at 655–660nm is absorbed by cytochrome c oxidase in the follicle's mitochondria. That absorption triggers measurably higher ATP output, reduced oxidative stress, and — crucially — longer growth phases. Stronger follicles produce thicker hair shafts. Over 20–26 weeks, that translates to visible regrowth.

Before 2018, the same technology existed only inside dermatology clinics, typically charging £200–£400 per session across a multi-month protocol. Most people gave up before the evidence base could show up in their mirror. The Harvard data legitimised at-home consumer devices that delivered the same wavelengths at clinical irradiance — and overnight, an £8,000 course of clinic visits became an £80 device you could use on the sofa.