We know. We hear it every day. Stress is bad—very bad—for us. It sends cortisol and adrenaline coursing through our bodies and, unchecked, causes adrenal fatigue, makes us age faster, and contributes to some pretty major health problems, such as heart disease. So why can't we just turn off the switch? Stanford's Robert Saplosky, who has studied stress in mammals for decades, addresses that question in tonight's National Geographic special airing on PBS. Watch a short intro and check out your local listings at the Stress: Portrait of a Killer website.