A beard grows because of hormones and genetics. Just like all other body hair on warm-blooded mammals, beards go through growth cycles. The hair follicles of beards are stimulated by various biological factors.
Hair growth is determined by biological signals from cells, blood and nerves. Human hair, including beards and other facial hair, is a non-living strand that goes through continuous cycles of growth stages called anagen, catagen and telogen. Anagen is the process of growth, catagen is degradation or shedding, and telogen is rest. Each follicle produces up to 20 hairs in its lifetime.
Beard growth is androgen-dependent, meaning that its growth depends upon testosterone levels and increases during male puberty. Beard development is also affected by thyroid hormones and other hormones. The hair fiber is made up of keratin, a protein. Beard color is determined by melanocyte cells, which contain melanin and are activated during anagen.