Your body responds to stress by releasing a cascade of hormones that put all body systems on full alert. In the short term, that provides a surge of energy, heightened awareness, and razor-sharp focus, so you can meet the deadline, say, or snatch a child from harm's way. But when the stress is intense or persists, those same hormones can debilitate you. "When you're at peak performance, just the nerve cells needed for the response are firing. But in protracted or very stressful situations, too many nerve cells fire at once. That's when you freeze," explains Esther Sternberg, M.D., director of the Integrative Neural Immune Program at the National Institute of Mental Health. "And a whole lot of small hassles can have the same effect as a major stressor."

Johns Hopkins University researchers recently measured the impact of stress on thinking in a group of about 1,000 men and women ages 50 to 70. They found that the most stressed subjects, as indicated by high salivary levels of the stress hormone cortisol, performed worse than their calmer counterparts on nearly every measure of cognitive function, including language, processing speed, hand-eye coordination, verbal memory and learning, visual memory, and the ability to plan and carry out tasks.

Stress from multitasking—juggling work with home obligations, for example—can be particularly mind-numbing. "You are asking the brain to do something it's not designed for," Sternberg says. The only solution, often, is to pull back. "You have to shut down and reboot or your body will do it for you," she says. "Americans have this macho idea that the longer and harder you work the better. But to be productive, you have to give your body a break. Sleep, ask for support, use your vacation time."