The more active you stay, body and brain, the more you signal your internal systems that you continue to play a vital role in your own survival and the survival of the species. This leads to a virtuous cycle in which key internal mechanisms continue to regenerate, allowing you to play a vital role and, when you do that, pushing the cycle on. By contrast, if you grow stagnant, physically and mentally, the system is signaled that you are calling it quits and it need not “waste” resources on your survival.