Legend has it Queen Marie Antoinette’s hair turned white overnight after she heard her death sentence, but we don’t need to go too far. Have you ever been going through a stressful time and suddenly you see a lot of new white hairs on your head?
Scientists recently found out how stress is linked to the whitening of hair. Here is how it works.
First of all, our hair follicles cycles between 3 main phases:
Anagen when the hair is growing; Catagen when the hair follicle regresses and Telogen when the hair follicle is in rest-mode, after which the hair falls and it starts a new anagen phase.  For our hair to grow and to have color we need two special cells: hair follicle stem cells (HFSCs), and melanocyte stem cells (MeSCs), respectively.
Our focus here is on the melanocyte stem cells, which are usually in rest-mode, also known as quiescence. When they are activated, they become mature melanocytes that produce the melanin to give color to our hair. [2,3] If those cells die, or stop producing melanin, our hair becomes white!
In the hair follicle we have two different regions: the hair bulge and the hair bulb. Two similar names, but pay attention because they are not the same!
The hair bulge is where the melanocyte stem cells are. When they become activated they migrate downwards to the hair bulb and there is where they produce the melanin to give color to the new hair. You can imagine that the bulge is where we have a reserve of cells, just waiting to be activated, migrate and produce melanin in the bulb. They are activated in early anagen phase, when the hair is growing. When you get to catagen phase, the mature melanocytes die (and this is why the tip of a hair that has fallen is white). The pool of remaining undifferentiated stem cells in the bulge are the ones that will initiate melanogenesis in future cycles. 
Ok, so where does stress come in all this?
Under stress, sympathetic nerves that are close to the hair bulge become activated releasing molecules of norepinephrine. They bind to β2 adrenergic receptors inducing the dormant melanocyte stem cells to differentiate, activate and migrate to the hair bulb, but so rapidly that it culminates in a permanent depletion of those cells in the bulge. 
So next time you see a white hair after exams week you will know there is indeed a scientific explanation to it, and it’s not just all in your head!
 Breitkopf T, Leung G, Yu M, et al. The basic science of hair biology: what are the causal mechanisms for the disordered hair follicle? Dermatol Clin 2013;31(1):1–19.
 Chang, C.-Y. et al. NFIB is a governor of epithelial-melanocyte stem cell behaviour in a shared niche. Nature 495, 98–102 (2013).
 Rabbani, P. et al. Coordinated activation of Wnt in epithelial and melanocyte stem cells initiates pigmented hair regeneration. Cell 145, 941–955 (2011).
 Nishimura, E. K. et al. Dominant role of the niche in melanocyte stem-cell fate determination. Nature 416, 854–860 (2002).
 Zhang, B., Ma, S., Rachmin, I. et al. Hyperactivation of sympathetic nerves drives depletion of melanocyte stem cells. Nature 577, 676–681 (2020). https://doi.org/10.1038/s41586-020-1935-3