Seasonal hair shedding is a form of telogen effluvium that occurs in some individuals at least once a year. The main time for seasonal shedding in the year occurs around September and a second minor shed can occur around March.
This has been confirmed by research studies done. This phenomenon is common in mammals who moult their fur/hair in the summer months and grow it for the winter. In humans, this is an evolutionary ‘left-over’ trait that has diminished. Circadian clock is the body’s internal clock that is biochemically run and synchronised with solar time with melatonin is one of the main chemical mediator. Clock (circadian) genes are expressed in hair follicles (HFs).
The genes can be modulated by both the central circadian system and some external factors, such as light, thyroid hormones and temperature. Several epidemiological studies found that patients with sleep disorders showed an increased risk of developing alopecia areata (a population based study done in 2018 in Korea). Furthermore, inflammatory cells and the cytokines produced are also under circadian control as shown by studies published in Science in 2013.
In our case studies, we have a patient whose alopecia areata flared up when she went on holiday westward to Mexico (7hrs difference). Another patient who travels to Australia every 6 months notices telogen shedding when she gets there and again when she returns back to England.