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Seasonal hair shedding
We, and much of the mammalian world, have two main shed seasons. The biggest shed occurs in early autumn and a smaller one in spring. Essentially we are replacing our summer coats with winter coats. Perhaps the best example is the arctic fox that sheds a pigmented summer coat and replaces it with a white coat (camouflage) for winter conditions. Much of the research on seasonal shedding and hair regrowth is conducted with mink (some studies from Australia on sheep and goats too). Researchers have looked at prolactin and melatonin levels among other factors as a potential regulator of the changes in hair growth. Giving mink melatonin, or reducing exposure to daylight, promotes shedding of the summer coat and the onset of the winter anagen growth stage. Possibly it is a similar situation for humans where changes in daylight hours modify the melatonin secretion rate from the pineal gland and this in turn affects hormones and/or their receptors in the skin.
The shed cycle in humans has not been investigated in much detail, but a few studies from the UK demonstrate the seasonal shed cycles. Although there is an increase in shedding during spring and fall, the number of hairs shed still falls within the generally recognized limits of 50 to 100 hairs a day. So in spring and fall the daily shed rate may be closer to 100 hairs a day and in summer and late winter the typical shed rate is closer to 50 hairs a day. However, the average rate of shedding does vary from person to person. Some people can be high shedders, but also have high rates of hair growth and replacement so they have no net hair loss. To understand your own seasonal hair shedding it is better to compare the hair shed rate in spring/fall to summer/winter within yourself rather than compare it to other people. Whether there are still seaonal shed cycles for humans living at the equator is not known - I would guess not if the changes are due to melatonin levels.
http://www.keratin.com/aa/aa015.shtml
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but wait.....
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Melatonin increases anagen hair rate in women with androgenetic alopecia or diffuse alopecia: results of a pilot randomized controlled trial.
Fischer TW, Burmeister G, Schmidt HW, Elsner P.
Department of Dermatology and Allergology, Friedrich-Schiller-University, Erfurter Strasse 35, D-07740 Jena, Germany.
tobias.fischer@derma.uni-jena.de
BACKGROUND: In addition to the well-known hormonal influences of testosterone and dihydrotestosterone on the hair cycle, melatonin has been reported to have a beneficial effect on hair growth in animals. The effect of melatonin on hair growth in humans has not been investigated so far. OBJECTIVES: To examine whether topically applied melatonin influences anagen and telogen hair rate in women with androgenetic or diffuse hair loss. METHODS: A double-blind, randomized, placebo-controlled study was conducted in 40 women suffering from diffuse alopecia or androgenetic alopecia. A 0.1% melatonin or a placebo solution was applied on the scalp once daily for 6 months and trichograms were performed to assess anagen and telogen hair rate. To monitor effects of treatment on physiological melatonin levels, blood samples were taken over the whole study period. RESULTS: Melatonin led to a significantly increased anagen hair rate in occipital hair in women with androgenetic hair loss compared with placebo (n=12; P=0.012). For frontal hair, melatonin gave a significant increase in the group with diffuse alopecia (n=28; P=0.046). The occipital hair samples of patients with diffuse alopecia and the frontal hair counts of those with androgenetic alopecia also showed an increase of anagen hair, but differences were not significant. Plasma melatonin levels increased under treatment with melatonin, but did not exceed the physiological night peak. CONCLUSIONS: To the authors' knowledge, this pilot study is the first to show that topically applied melatonin might influence hair growth in humans in vivo. The mode of action is not known, but the effect might result from an induction of anagen phase.
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