Murine epidermal stem cells undergo alternate cycles of dormancy and activation, fuelling tissue renewal. However, only a subset of stem cells becomes active during each round of morphogenesis, indicating that stem cells coexist in heterogeneous responsive states. Using a circadian-clock reporter-mouse model, here we show that the dormant hair-follicle stem cell niche contains coexisting populations of cells at opposite phases of the clock, which are differentially predisposed to respond to homeostatic cues. The core clock protein Bmal1 modulates the expression of stem cell regulatory genes in an oscillatory manner, to create populations that are either predisposed, or less prone, to activation. Disrupting this clock equilibrium, through deletion of Bmal1 (also known as Arntl) or Per1/2, resulted in a progressive accumulation or depletion of dormant stem cells, respectively. Stem cell arrhythmia also led to premature epidermal ageing, and a reduction in the development of squamous tumours. Our results indicate that the circadian clock fine-tunes the temporal behaviour of epidermal stem cells, and that its perturbation affects homeostasis and the predisposition to tumorigenesis.Epidermal stem cells ensure that skin homeostasis is maintained. Murine epidermal stem cells are located either at the permanent portion of the hair follicle-termed the bulge-and are exclusively responsible for hair cycling [1][2][3][4] ; or at the junction between the epidermis and the hair follicle (isthmus), and feed into the epidermis and sebaceous glands [5][6][7] . In addition, a continuous proliferation of basal interfollicular epidermal cells ensures daily epidermal maintenance 8 .Bulge stem cells undergo bouts of activation followed by periods of dormancy, to establish hair follicle cycling. Robust TGF-b and Bmp signals act as 'activation breaks', rendering bulge cells dormant during the resting phase of the hair cycle (telogen) [9][10][11] . At the onset of the growth phase (anagen), bulge cells respond to Wnt signals by migrating into the lower proliferative hair germ region, where they contribute to follicle growth [12][13][14][15] . Subsequently, at mid-anagen, the bulge undergoes a second round of activation, which replenishes cells lost at the onset of anagen 2,3 . However, the response of bulge stem cells to activating stimuli is a heterogeneous process, as only a subset of them become active during either stage of activation 12,13 . The nature of such niche heterogeneity is currently unknown. Importantly, perturbing the equilibrium between the responsive and non-responsive stem cell states causes tissue malfunction and increases the risk of carcinogenesis [16][17][18][19][20] .Here, we analysed the role of the molecular clock in fine-tuning the function of epidermal stem cells. The mammalian clock machinery anticipates and synchronizes vital functions related to the physiological circadian needs of the organism 21 . The core molecular clock is established by a positive limb, composed of heterodimers of the transcription fa...