You just swore at a guy who cut you off on the way to work. Your children have taken to calling you the Wicked Witch. You start daydreaming about cocktail hour shortly after lunch. And your mind always seems to be in a whirl. What’s your problem? In a word: stress.

You’re not alone. Nearly half of all Americans are concerned about the level of stress in their lives, according to the American Psychological Association’s 2006 Stress Survey. Chalk it up to “our overscheduled, harried 21st-century lifestyle, which can wreak havoc with our relationships and our work,” says Bruce S. McEwen, M.D., a coauthor of The End of Stress as We Know It (Joseph Henry, $20, www.amazon.com).

Not to mention our health. Chronic stress weakens the immune system and increases the risk for a range of illnesses, including heart disease and depression. Stress drives people to eat too much, sleep too little, skimp on exercise, and shortchange fun. It doesn’t have to be toxic; a little stress can sharpen focus, improve memory, and heighten emotions. But sometimes good stress goes bad, and researchers have just begun to figure out how. “By understanding what makes it go wrong,” says McEwen, “we have the power to make it right.”