A 2010 study revealed that a decrease in T explained much of the loss in phase III and IV sleep, or "deep sleep" that is characteristic of aging males.  At age 50, men spend 5-7 percent of their sleep time in Phase III and IV and by age 60 it is nearly zero or nonexistent. Young men, on the other hand, spend 10-20 percent of thier total sleep time in these stages due to neuronal integrity and abundant testosterone levels..
Primary hypogonadism (congenital or acquired): Testicular failure due to diseases and conditions in the body such as cryptorchidism, bilateral torsion, orchitis, vanishing testis syndrome, orchiectomy, Klinefelter Syndrome, chemotherapy, or toxic damage from alcohol or heavy metals; these men usually have low serum testosterone levels and gonadotropins (FSH, LH) above normal range Hypogonadotropic hypogonadism (congenital or acquired): Gonadotropin or luteinizing hormone-releasing hormone (LHRH) deficiency or pituitary-hypothalamic injury from tumors, trauma, or radiation; these men have low testosterone serum concentrations but have gonadotropins in the normal or low range.
What researchers have found is that individual hair follicles have different expression of genes within the follicle. Each gene expression reacts differently to androgen. Some genes inhibit follicle health and some increase follicle health. Since each follicle is independent of one another, each gene expression is also independent of one another. This is why hair transplants work. The follicles on your head might be dying, but the ones in other areas of your body are not, so doctors can simply move them. The transplanted hair follicle will not die because the genes associated with that follicle are not negatively affected by androgen, no matter what area of the body they’re in. Nothing like back hair on your head! The process that allows a specific gene to be expressed in a certain follicle isn’t yet understood. What is known is that the programming occurs in the pattern processing during development.