It all boils down to cortisol: Come bedtime, you experience a drop in the infamous stress hormone (and an uptick in melatonin, which has some impressive benefits for skin cell repair), but without proper shut-eye, your body is more likely to increase cortisol levels, which, if the poor sleep is chronic, can snowball pretty quickly. In other words, a lack of sleep makes us stressed—and this can lead to stress-induced hair loss.The link between stress and hair loss is well-documented—so much so, it has its own diagnosis: a medical condition known as telogen effluvium. The stress pushes the follicles into a dormant phase (known as the telogen phase), which can cause those hairs to fall out. It’s not usually permanent (more in a moment), and it’s incredibly common. Time and again we talk about how important managing stress is for hair care, but you might not trace the origin back to poor sleep. Of course, lack of zzz’s might not be the only factor contributing to stress, but let’s not put it on the back burner, either.