Even though reflectance spectroscopy has shown a potential for the evaluation of hair growth, further investigations need to be performed to determine whether the method itself has an effect on the hair follicle cycle. It is worth noting that the absorption peaks of spectra change with the hair follicles’ cycling, and even disappear when the reflectance spectra are at a relatively low level. This is because the wavelength range corresponds to the absorption peaks of blood. At later anagen and early catagen stages, light can hardly penetrate into the dermis because of the strong absorption of the concentrated melanin pigment, so it is impossible to detect the information from blood vessels located in dermis. In contrast, when hair follicles are at the later catagen, telogen, and early anagen stages, the melanin content in the skin epidermis and hair shaft is much lower. More light can enter into the dermis and be absorbed by blood, so the characteristic peak of blood can be clearly demonstrated in the reflectance spectra. In addition, blood perfusion may be related to the hair follicle cycling. Angiogenesis happened during the synchronized switches of the hair follicle from telogen to anagen, and the degeneration of capillary loops was noted during the catagen stage.23,24 However, with the increase in absorption of melanin, reflectance spectroscopy is insufficient to describe the relationship of blood perfusion and the cycle of hair follicle, therefore, further investigation should be performed in the future.