We spend one-third of our lives doing it—or at least we should. Sleep bolsters our immune system, and a deficit of it has been linked to such catastrophic health consequences as cancer, Alzheimer’s disease, and heart failure. It’s crucial for proper brain functioning, enhancing our ability to learn, memorize, and decide. As we’ll explore, it’s also of paramount importance to bodybuilding, affecting everything from strength and testosterone to pain sensitivity. Yet sleep is something you’re probably not getting enough of to optimize your body, inside and out.

More than 750 scientific studies demonstrate the positive relationship between sleep and athletic performance. Here are just some of the many things known. Sleep for fewer than eight hours nightly, especially fewer than six, and your time to physical exhaustion drops by up to 30% with a similar reduction in aerobic output. Your metabolic, respiratory, and cardiovascular capabilities are hampered. Lactic acid builds up faster. The ability to cool yourself via sweating is impaired. Motor skills are reduced. One study showed the odds of athletic injury were nearly doubled by a two-hour sleep deficit. All of the above can wreck a workout, and that’s before we even address strength. What’s more, research points to the final two hours of an eight-hour sleep—a phase when the brain is putting it all together, completing the physical and mental tune-up, so to speak—as critical to athletic performance.