To learn how you lose hair, you first have to understand how it grows. Hair goes through its own growth cycle that's unrelated to seasons or hormones or anything else. It's a random biological process that's dictated largely by your genetic disposition. The two main phases:
Anagen (active): Cells in the root are dividing quickly and pushing the hair out. It averages to three years.
Telogen (resting): This phase lasts for about 100 days on the scalp. Consider it hair hibernation -- the follicle is completely at rest.
Doctors don't know why certain hair follicles are programmed to have a shorter growth period than others. One suspected factor for age-related male-pattern baldness is a person's level of androgens—the "male" hormones that are actually produced by both men and women. Take a look at Figure 2.2. For many years, people believed that a predominance of testosterone was the root cause of baldness, but it's not quite that simple. We do know that we lose hair especially fast if it is exposed to dihydrotestosterone (DHT, which comes from metabolism of testosterone). It's believed that the exposure of follicles to levels of testosterone that are normal for adult males causes the hair follicles to go into a resting state. This DHT is formed in the testes, prostate, adrenals, and hair follicles themselves through an enzyme called 5-alpha reductase. The enzyme raises the levels of DHT, and that's why there's a link between higher levels of this enzyme and areas of baldness. DHT changes healthy follicles to follicles that grow thin dwarf hairs—hairs that resemble peach fuzz. Essentially, DHT shrinks hair follicles, making it impossible for healthy hair to survive. Drug companies have targeted this process by making antibaldness medication that inhibits 5-alpha reductase, the enzyme that makes DHT. (Some of the infrequent side effects of these meds include impotence, decreased libido, and breast enlargement.)
Now, age-related baldness isn't the only reason why clumps of hair start falling from the head like raindrops from the sky. Other causes, especially for women, include low iron levels and anemia (low blood count), recent anesthesia for surgery (it's the stress of the surgery and the pressure on one area of the head, not the anesthesia), menopause or being postpartum, autoimmune diseases such as lupus, thyroid disease, and polycystic ovarian disease (PCOS).