Our hair grows in cycles. At any one time, about 85% of hair is in the growth phase and 15% in the rest phase. The reason for this is simple. While it might be neat if all of the hair on the head was simultaneously in the growth phase - that would likely mean that at some point, all hair would be in the rest phase, after which those hairs would be shed. That means that everyone would, for a short time at least, be completely bald.
All hair grows from hair follicles, which cover almost all of the body (the exceptions, thankfully, are the palms of the hands and soles of the feet). There are approximately 100,000 follicles on each head, each of which grows many hairs over a lifetime. For most people of average lifespan, a single follicle will grow about twenty new hairs.
Follicles go through growth and rest cycles, which vary with each individual. Each follicle will experience many of these cycles over the course of a life. The growth cycles average from 2 to 7 years and the rest cycles from 2 to 3 months. These cycles also vary according to the part of the body the follicle is located on.
Some follicles on the head will continue to produce hairs throughout the lifetime of the person, even for those who live in excess of 100 years. The individual hairs are produced in the bulb, which is located at the base of the follicle. This bulb also produces the pigment that colors the hair. Less pigment is produced as people get older which is why we get gray and white hair.
There are actually three phases in the hair growth cycle. They are the anagen, catagen, and telogen phases. The anagen phase is the growth phase, and can last as much as 7 years. The catagen phase occurs between the anagen and the telogen phase and only lasts a few weeks. This is an intermediate phase. The telogen phase is the rest phase and lasts a few months, after which the hair falls out. The length of the anagen phase determines how long a person's hair can be. Since hair grows about ½" a month, a person whose hair is in the growth phase for close to 7 years will have hair more than 3' long.
Hair follicles will keep producing hair under most circumstances, and will almost never stop doing so. There are however some conditions that can cause hair to stop growing. Some serious medical conditions can affect hair growth. Illness such as severe anemia can cause a stoppage of hair growth, as can malnutrition and a poor diet. Some hormonal conditions (thyroid problems) can cause hair growth to be stopped, as can certain vitamin and mineral deficiencies.
Of course, the most common cause of hair loss in men and women is pattern baldness. In men, this is seen in the horseshoe shaped ring of hair around the sides and back of the head. In women, the hair tends to thin over the entire head. This condition is caused by the conversion of the male hormone testosterone into DHT by an enzyme in the body. DHT acts on the follicles to cause them to shrink and produce finer and finer hair - and eventually stop producing them. There are treatments available for this condition that can stop the loss in many people, and often re-grow hair as well. It must be continued for life or all new hair is lost, and hair loss resumes.
Hair growth is a complex process that is not fully understood by scientists. Research continues in an effort to better understand it, and to help those whose growth functions have been adversely affected in some way.
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