Many animals experience a “shedding season” where their fur or coat changes dramatically to adapt to environmental shifts. This raises a question: Do humans have a similar, noticeable hair shedding season, or are human hair loss patterns driven by different mechanisms?
Understanding the Human Hair Cycle
Human hair growth and shedding follow a continuous biological process with distinct phases. Each hair follicle operates independently, meaning hairs are always in different stages of their life cycle. The anagen phase is the active growth period, lasting two to eight years, with 85% to 90% of scalp hairs typically in this phase.
Following growth, hair enters the catagen phase, a short transitional period lasting two to three weeks, during which the follicle shrinks. The telogen phase is a resting period of two to four months, where hair remains in the follicle but does not grow. Finally, the exogen phase is when hair is released from the scalp and sheds.
The Reality of Human Seasonal Shedding
Research suggests humans may experience subtle seasonal variations in hair shedding, though not as dramatic as animal molting. Some studies indicate a slight increase in hair loss during late summer and early autumn. This pattern might be linked to a higher proportion of hairs entering the resting (telogen) phase during summer months, with shedding occurring about 100 days later.
The precise reasons for this subtle seasonal increase are not fully understood. One theory suggests increased summer sunlight exposure could influence the hair growth cycle, prompting more hairs to enter a resting state. Another hypothesis proposes humans may grow more hair in summer to protect the scalp from intense sun, shedding this additional growth in cooler months.
Factors Influencing Hair Shedding Beyond Seasonality
Factors unrelated to seasons can influence hair shedding. Stress is a common trigger, leading to telogen effluvium, where more hairs than usual enter resting and shedding phases simultaneously. This can result in increased hair loss months after a stressful event.
Hormonal changes also play a significant role. Pregnancy, childbirth, menopause, and thyroid imbalances can disrupt the hair growth cycle, leading to increased shedding. Nutritional deficiencies, particularly those involving iron, protein, zinc, or vitamin D, can also impact hair health and contribute to shedding. Certain medications and genetic predispositions, such as androgenetic alopecia, also influence hair loss patterns.
Recognizing Normal Versus Excessive Shedding
It is typical to shed between 50 and 100 hairs daily as part of the natural hair cycle. This amount can vary based on hair length and thickness, with longer hair often making shed hairs more noticeable.
If hair loss significantly exceeds this range, or if other noticeable changes occur, it may be considered excessive. Indicators like visible thinning, bald patches, a widening hair part, or persistent clumps of hair suggest a need for professional evaluation. A dermatologist can help determine the underlying cause and appropriate course of action.