Sapolsky's Why Zebra’s Don’t Get Ulcers: a charming tour of stress science

Obviously, if you don’t exercise at all, it is not good for you. Exercise improves your health. And a lot of exercise improves your health a lot. But that doesn’t mean that insanely large amounts of exercise are insanely good for your body. At some point, too much begins to damage various physiological systems. Everything in physiology follows the rule that too much can be as bad as too little. There are optimal points of allostatic balance.