The hair cycle describes the events that occur when human hair grows. Every single hair on the human body is continuously moving through the hair cycle, some at different rates than others, but the chain of events that make up this series is consistent.
Anagen is sometimes known as the growing phase. During anagen, the bulb of the hair multiplies rapidly to produce thousands of cells which are pushed upwards from the base. These cells are compressed into a tight column of many layers and subsequently keratinise to form the hair follicle. Melanocytes which surround the bulb will also release melanin into the growing hair at this stage, and the colour of the melanin determines your individual hair colour.
This phase varies depending on the location of the hair. On the scalp, it typically lasts between 3 and 5 years and grows 1cm of hair each month. The eyebrows on the other hand go through the growth phase in 4 months and will grow almost half a centimetre per month.
The maximum length that an individuals hair can reach differs between people due to the variability of their anagen phase length. This is why, for example, some people find that they can grow their hair to waist length whereas others cannot get past shoulder length.
Catagen is sometimes referred to as the transition phase of the hair cycle and it signal the end of the growth phase. The growing hair bulb ceases production and no new hair is produced. Signals are sent to a reserve of stem cells within the dermal papilla, which differentiate to form the replacement hair bulb.
The telogen phase is referred to as the resting phase as the existing hair undergoes no changes. The old hair bulb is no longer productive and the old hair shaft is shed, allowing space for the new hair to replace it.