Maybe if I were less active, I would have developed RSI after 5 years instead of 13. Perhaps our amount of exercise and other physical activity is not enough to counteract the 8-hour daily dose of static computer posture. Certainly, I exercise much less than I type. I used to be very active, playing volleyball and other sports all the time, but haven't kept up with it lately. I think that's part of the reason my shoulders got so weak.
Sleep is restorative.
For the first two years of my RSI symptoms, a good night's sleep would alleviate them, seductively resetting my personal doomsday clock so I could go out and tear up my arms again another day. The most important part about sleep is simply that it's eight hours during which you're not typing. But beyond that, your tissues can recuperate and regain some of their normal flexibility and stamina. As far as the early onset of RSI for an individual, stamina is the key. If you get enough sleep, your shoulder muscles have the stamina to keep your upper torso from collapsing into the nerve network that innervates the arms.