Seasonal hair loss: what is happening?

The skin naturally possesses hair follicles. The head contains approximately 1 million hair follicles, capable of producing up to 150,000 hairs depending on the individual. Originating from a hair follicle, the life of a hair alternates through different phases:

• The Anagen Phase (2 - 6 years): This is the growth phase of the hair during which there is proliferation of the matrix cells that form the inner sheath of the root, the cortex, and the medulla of the hair shaft. The synthesis and pigmentation of the hair shaft only occur during this phase. Its duration determines the length of the hair, generally 1 cm per month.

• The catagen phase (2 - 3 weeks): The first sign of the catagen phase is the cessation of melanin production, the pigment responsible for the colour of our hair, in the hair bulb. The hair stops growing but remains attached to its hair follicle.

• The telogen phase (2 - 3 months): After the catagen phase, the follicles rest in a dormant phase, the telogen phase. The hair shaft eventually detaches from its follicle, which is already starting to produce a new hair beneath the skin.

Thus, within a head of hair, not all hairs are necessarily in the same phase as they each have their own life cycle. Some may be in a growth phase, while others are about to shed. This is why we lose approximately 50 to 100 hairs per day. This shedding is normal and does not impact the appearance and density of the hair. The number of hair renewal cycles during an individual's lifetime ranges between 12 and 30. However, their life cycles are influenced by hormonal or nutritional variations.

You may have noticed that with certain seasonal changes, your hair loss is more significant than usual.

A study was conducted on 14 men to assess the evolution of the proportion of hair in the anagen phase over the course of a year. The proportion of follicles in the anagen phase was evaluated from the roots of plucked hairs and observed under a microscope. The data shows that the proportion of scalp follicles in the anagen phase fluctuated seasonally. A peak of 90% of follicles in the anagen phase was observed around March, and steadily declined until September. Hair loss was at its maximum during the August/September period, when the fewest follicles were in the anagen phase. Indeed, during this period a 20% increase in follicles in the telogen phase was observed.

The transition from summer to autumn or from winter to spring is marked by a change in light and climate. These factors influence melatonin, a hormone produced by the pineal gland that is inhibited in the presence of light. It is involved in the hair cycle by increasing the anagen phase. Thus, from March to September, melatonin is more likely to be inhibited by the sun. After the summer, the level of melatonin is at its lowest and the hair enters the catagen phase and then the telogen phase. This explains the increase in the number of follicles in the telogen phase in August/September observed in the study. This phase lasts 2 to 3 months and ends with the hair detaching from its follicle. It is for this reason that a greater hair loss is observed with the arrival of autumn.

Winter is also synonymous with stress and fatigue, which influences the rhythm and speed of the hair life cycle. Indeed, stress promotes the transition to the catagen phase which tends to accelerate the hair life cycle and promote its fall. This is why some people may notice a higher hair loss with the onset of winter.

RANDALL V. A. & al. Seasonal changes in human hair growth. British Journal of Dermatology (1991).

COURTOIS M. & al. Periodicity in the growth and shedding of hair. British Journal of Dermatology (1996).

FISCHER T. W. & al. Melatonin increases anagen hair rate in women with androgenetic alopecia or diffuse alopecia: results of a pilot randomized controlled trial. British Journal of Dermatology (2004).