The hair life cycle consists of 3 repeating phases. The phase of growth (regeneration), the phase of rest (degeneration) and the phase of apoptosis. During laser hair removal, only hair that is in the phase of growth will be permanently removed, since at that time the hair follicle has a higher melanin concentration and correspondingly laser energy is better absorbed. The rest of the hair grows again; hence, secondary sessions are required until the laser reaches all the hair at the stage of its life cycle which is appropriate for permanent removal.