Melatonin gets sold for sleep. So why would you rub it on your scalp? Fair question. The mechanism turns out to be more interesting than the marketing. Melatonin receptors (MT1 and MT2) are expressed on dermal papilla cells, and exogenous melatonin extends the anagen phase in mouse models. Fischer's group in Germany ran the first proper human trial back in 2004, applying a nightly 0.1% scalp solution to women with diffuse hair loss. After six months, hair density measurably improved versus vehicle. Small effect, but it was real and reproducible.

A 2012 review pooled data from five small trials totalling around 190 patients. The pattern held: a consistent 8 to 10 additional hairs per cm squared over six months versus placebo. Nowhere near minoxidil's 15 to 20 hairs, but a third of that response from a compound with essentially no systemic side effects when used topically. The mechanism appears to be dual. Direct receptor signalling on follicle cells extends anagen, and melatonin acts as a free radical scavenger in perifollicular tissue. Oxidative stress is increasingly thought to contribute to follicle miniaturisation, especially in chronic stress states.

Where this gets interesting is the question of stacking. Melatonin and minoxidil work through completely different pathways, so combining them shouldn't produce mechanism conflict. A 2015 Hardman paper showed melatonin pretreatment preserved follicle stem cell function under oxidative stress in ex vivo human follicle culture. No formal human stacking trial has been done. For patients already tolerating minoxidil, adding a nightly topical melatonin solution is plausible as a low-risk add-on with possible additive benefit. Application close to bedtime can cause slight drowsiness in some users, which is the only practical caveat worth mentioning.