Those who get runner’s high know it when they feel it. But a clinical definition is a little more slippery. “I remember someone saying the runner’s high was the moment when the body was disconnected from the brain,” says Francis Chaouloff, who studies running and motivation in mice at the French Institute of Health and Medical Research in Bordeaux. This sense of extreme euphoria, he says, is generally limited to people running or exercising for long periods of time, over many miles or hours.