But others have a milder definition. “We use the [phrase] as a way to capture a pretty broad range of psychological changes,” says David Raichlen, a biological anthropologist at the University of Arizona in Tucson. “I look at it at one level as just an improved general feeling of well-being that encompasses mood and…positive neurobiological responses.” This could mean euphoria from a half-marathon, yes, but it also could include the more minor positive feelings from a brisk 30-minute jog or even a walk.