As teens progress through puberty, unprecedented growth occurs in body and brain that requires a lot of sleep. (npr.org)
Likewise, the signals between body and brain that make people feel thirsty may not function as well in old age. (scientificamerican.com)
Sleep is # 1 on my list because without sleep, you're running in a restoration deficit for all of your normal body and brain functions. (thebettyrocker.com)