It is known that baldness caused by androgenetic alopecia is involved with androgen and the androgen receptor. Furthermore, it has been reported that testosterone secretion follows a circadian rhythm. Therefore, we hypothesized that a relationship exists between androgen-induced alopecia and biological rhythm. The mammalian circadian rhythm is controlled by several clock genes. Brain and muscle aryl hydrocarbon receptor nuclear translocator-like protein-1 (BMAL1), one of the clock genes, is a transcription factor that plays central roles in the regulation of circadian rhythms. In this study, we investigated the influence of BMAL1 on hair follicle functions and hair growth. Mice deficient in BMAL1 expression exhibited a delay in hair regrowth after shaving. In hair follicles of mouse vibrissa, expression of Bmal1 and other clock genes was found to be rhythmic. Knockdown of BMAL1 in human follicle dermal papilla cells resulted in modulation of expression of several hair growth-related genes. Therefore, we concluded that expression of clock genes in hair follicles is linked to the circadian rhythm and that BMAL1 can regulate hair growth.