It leaves you feeling euphoric.
Yes, that "runner's high" really does exist if you're willing to shift into high-intensity mode. Ratey recommends sprint bursts through interval training. Run, bike, or swim as fast as you can for 30 to 40 seconds and then reduce your speed to a gentle pace for five minutes before sprinting again. Repeat four times for a total of five sprints. "You'll feel really sparkly for the rest of the day," he says.