In today’s fitness space, self-experimentation is the name of the game. All kinds of people are embracing new technologies and diets in the hope of finding faster strategies for getting in the best possible shape. In this crowd, few are pushing things further than Ben Greenfield. The exercise physiologist and personal trainer has made his mark by exploring the limits of what seems reasonable (Example A: injecting his penis with stem cells) and voicing controversial ideas, including skepticism about standard vaccination practices. In his new book, Boundless: Upgrade Your Brain, Optimize Your Body & Defy Aging, he covers almost anything you might want to know about being the fittest and healthiest you can be. For this episode, Outside editor Chris Keyes speaks with Greenfield about strategies for better sleep, the upsides of cold therapy, the problems with gym workouts, and more.