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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 may be 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 entirely due to melatonin levels.
Seasonal
hair shedding references
- Johnston B, Rose J. Role of prolactin in regulating
the onset of winter fur growth in mink (Mustela vison): A reconsideration.
J Exp Zool. 1999 Sep 1;284(4):437-44.
- Courtois M, Loussouarn G, Hourseau S, Grollier
JF. Periodicity in the growth and shedding of hair. Br J Dermatol.
1996 Jan;134(1):47-54.
- Parry AL, Nixon AJ, Craven AJ, Pearson
AJ. The microanatomy, cell replication, and keratin gene expression
of hair follicles during a photoperiod-induced growth cycle in
sheep. Acta Anat (Basel). 1995;154(4):283-99.
- Nixon AJ, Choy VJ, Parry AL, Pearson AJ.
Fiber growth initiation in hair follicles of goats treated with
melatonin. J Exp Zool. 1993 Sep 15;267(1):47-56.
- Randall VA, Ebling FJ. Seasonal changes
in human hair growth. Br J Dermatol. 1991 Feb;124(2):146-51.
- Rose J, Oldfield J, Stormshak F. Apparent
role of melatonin and prolactin in initiating winter fur growth
in mink. Gen Comp Endocrinol. 1987 Feb;65(2):212-5.
- Rose J, Stormshak F, Oldfield J, Adair
J. Induction of winter fur growth in mink (Mustela vison) with
melatonin. J Anim Sci. 1984 Jan;58(1):57-61.
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