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If stress is supposed to make you sick, why do so many people fall apart after the big project or final exam? Marc Schoen, PhD, an assistant clinical professor at the UCLA School of Medicine and the author of When Relaxation is Hazardous to Your Health, calls this vacation-ruining phenomenon the letdown reaction. He blames prostaglandins—hormonelike substances that are released along with stress hormones and have been linked to ailments like heart disease, cancer, arthritis, and migraines. Stress hormones put the immune system on alert so the body is in fighting form; when stress subsides and immune cells retreat, the body becomes vulnerable to the prostaglandins' dangerous effects. Schoen's prescription: "Don't go from 100 miles and hour to a screeching halt."

Instead of immediately crashing beside the pool or on the couch, schedule brief bursts of exercise. "Just five or six minutes at a time, several times a day, is enough to enhance the immune system," he says. Stimulating your brain is also effective. Schoen suggests doing crossword puzzles or playing a card game like Concentration, ideally against the clock. After a day or two of "slow relaxation," he says, you can veg out completely without suffering any ill consequences.

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