The three growth cycles of human hair are anagen (active growth), catagen (transition), and telogen (rest). Every hair on your body moves through these three phases independently, which is why you don’t shed all your hair at once. At any given moment, the vast majority of your scalp hairs are in the growth phase, a small fraction are transitioning, and about 9% are resting before they fall out. This continuous cycling is what keeps a relatively stable head of hair while still replacing old strands with new ones.
Anagen: The Growth Phase
Anagen is the longest and most active phase. During this stage, cells in the hair root divide rapidly, building the hair shaft and pushing it upward through the follicle. Scalp hair grows at roughly 1 centimeter per month, or about 6 inches per year, adding up to a normal daily growth rate of around 0.35 millimeters.
How long a follicle stays in anagen is what determines how long your hair can get. For scalp hair, the anagen phase lasts two to eight years, depending largely on genetics. That wide range explains why some people can grow hair past their waist while others find their hair plateaus at shoulder length no matter what they do. Eyebrow hair, by comparison, stays in anagen for only two to three months, which is why eyebrows stay short even if you never trim them. The same principle applies to arm hair, leg hair, and eyelashes: shorter anagen phases mean shorter maximum lengths.
In a healthy scalp, the ratio of hairs in anagen to hairs in the resting phase is roughly 12:1 to 14:1. That means the overwhelming majority of your hair, around 85% to 90%, is actively growing at any given time.
Catagen: The Transition Phase
Catagen is the briefest phase, lasting about two to three weeks. When a follicle enters catagen, it stops producing new hair cells. The lower portion of the follicle shrinks and detaches from its blood supply, which had been delivering nutrients to fuel growth. The hair strand itself doesn’t fall out yet. Instead, it becomes what’s sometimes called a “club hair,” anchored loosely in the shrinking follicle while the follicle prepares to enter its resting stage.
Only about 1% to 3% of your scalp hairs are in catagen at any given time, so this phase is essentially invisible in day-to-day life. Think of it as the off-switch between active growth and rest.
Telogen: The Resting Phase
During telogen, the follicle sits dormant for roughly two to four months. The old club hair remains loosely in place while, deep inside the follicle, the early signals of a new hair begin forming. About 9% of your scalp hairs are in telogen at any time, which translates to the 50 to 150 hairs you normally shed each day. These hairs fall out during brushing, washing, or simply from the gentle friction of a pillowcase, and that level of shedding is completely normal.
Some researchers describe an additional sub-phase called exogen, which refers to the moment the old hair actually releases from the follicle. Whether exogen is considered its own phase or simply the tail end of telogen depends on who you ask, but the practical result is the same: the old strand falls out and a new anagen hair begins growing in its place.
Why Hair Length Varies Across the Body
The reason your scalp hair can reach your shoulders while your arm hair stays a quarter inch long comes down entirely to anagen duration. Scalp follicles are genetically programmed for two to eight years of active growth. Eyebrow follicles cycle through their entire growth period in about four months. Since the growth rate is roughly the same everywhere on the body, a shorter growing window simply means a shorter maximum length. This is also why individual scalp hairs eventually stop growing and fall out even if you never cut them. They’ve reached the end of their anagen clock.
What Signals the Follicle to Switch Phases
The transition between phases isn’t random. It’s controlled by chemical signals exchanged between the follicle and surrounding tissue. The most important of these is the Wnt signaling pathway, which acts as the primary trigger for pushing a resting follicle back into active growth. When Wnt signaling is blocked experimentally, follicles can’t enter anagen at all. When it’s amplified, follicles shift into growth mode earlier than expected.
Working alongside Wnt is a counterbalancing signal called BMP, which peaks during the transition and resting phases and stays low during active growth. These two systems essentially form an on-off switch: Wnt pushes the follicle toward growth, while BMP keeps it in check. A third pathway, Hedgehog signaling, also helps nudge resting follicles into the growth phase. The interplay between all of these signals determines how smoothly your hair cycles and how quickly your follicles re-enter anagen after shedding an old hair.
When the Cycle Gets Disrupted
The most common hair-cycle disruption is telogen effluvium, a condition where a large number of follicles get pushed from anagen into telogen all at once. Instead of the normal 9% of hairs resting, the proportion jumps significantly, leading to noticeable thinning and shedding two to three months after the triggering event.
Common triggers include high fevers, severe infections, major surgery, crash dieting, low protein intake, iron deficiency, thyroid problems, and hormonal shifts after childbirth. Certain medications can also cause it, particularly beta-blockers, blood thinners, high-dose vitamin A, and some anti-seizure drugs. The good news is that telogen effluvium is almost always temporary. Once the underlying stressor resolves, follicles gradually cycle back into anagen, and hair density returns to normal over six to twelve months.
Understanding the three-phase cycle makes this kind of shedding less alarming. The hair that falls out during telogen effluvium was healthy hair that was simply forced into the resting phase early. The follicles themselves are undamaged and fully capable of producing new growth once the cycle resets.