It sounds crazy, but Autonomous Sensory Meridian Response — a phenomenon known more commonly as ASMR — has done wonders for my productivity levels. Basically, it's a super relaxing, full-body "buzz" that's produced by certain triggers, which differ according to the individual. Whether it's tapping sounds, crinkling sounds, or whispered voices, those who experience ASMR swear by it, and I'm one of them.
While it's also great for helping me get to sleep at night if I happen to be having trouble resting, during the day, plugging in my headphones and listening to some of my favorite ASMR artists on YouTube allows me to get in a zone of total productivity.
While the science behind ASMR isn't totally understood just yet, it works for me.