Exercise. Relatively new discovery that lactic acid, that comes from muscle use, is a "pre-cursor" for motivation.
Always question advice that people like to rattle off - humans are very good at perpetuating bad memes without a second thought. Like that running a marathon (26 miles) in one go is somehow healthy, without ever questioning why a healthy activity would make them shit their pants and nipples bleed.
In fact exercising would probably decrease the probability that I would be productive as it would consume more of my time and energy.
My 0.02