" When we reach stage 4 sleep, our bodies release a growth hormone referred to by the acronym GHRH (for growth hormone–releasing hormone). GHRH helps bruises and cuts, and fights off infections at the cellular level. GHRH stimulates the repair of cells, the growth of replacement cells, and, in children and adolescents, the creation of the new cells their bodies need to grow. GHRH also induces sleepiness. One reason fast-growing teenagers need so much sleep is that their GHRH levels are higher than their parents’ or grandparents’, and in laboratory experiments GHRH has been shown to help people with sleep problems get a better night’s rest. Conversely, a lack of sleep can inhibit cellular repair and growth. There’s evidence that long-term sleep deprivation can stunt growth. "

― Alex Soojung-Kim Pang , Rest: Why You Get More Done When You Work Less