An interesting article was published showing increased shedding of hair in women in summer months. This is probably true for males as well. Below is an abstract of this article.

Dermatology. 2009;219(2):105-10. Epub 2009 Apr 29. Links
Seasonality of hair shedding in healthy women complaining of hair loss.
Kunz M, Seifert B, Trüeb RM.
Department of Dermatology, University Hospital of Zürich, Zürich, Switzerland.

BACKGROUND: A number of otherwise healthy women with or without clinical alopecia complain of recurrent hair loss, presumably reflecting seasonality in the growth and shedding of hair. Objective: To test the hypothesis that periodicity in hair shedding reflects seasonal changes in human hair growth. METHODS: Retrospective case study over a period of 6 years of apparently healthy women with the complaint of hair loss. All underwent biochemical investigations, and trichograms were made. RESULTS: After exclusion of patients with a disease or on drugs known to cause hair loss, 823 women remained. Analysis of trichograms demonstrated annual periodicity in the growth and shedding of hair, manifested by a maximal proportion of telogen hairs in summer. A second peak seems to exist, though it is less pronounced, in spring. The telogen rates were lowest in late winter. CONCLUSIONS: These results confirm the findings of former authors who have indicated seasonal changes in human hair growth, though this is the first study performed systematically in a representative number of women.

Copyright 2009 S. Karger AG, Basel.