Stress sets off several physiological reactions in the body that can affect the skin. It causes the release of hormones like cortisol that thicken hair follicle cells and increase oil production—the perfect recipe for acne. Stress can also trigger neuropeptides, chemicals unleashed from nerve endings in the skin that leave it red or itchy, and encourage T cells (the skin's infection fighters) to overreact, making the skin turn over too quickly and flake or scale. Then there are the blood vessels: Under stress they become more reactive, either clamping shut (so skin looks pale or sallow) or opening too widely (causing the skin to flush).

When you're stressed, the skin's repair mechanisms are compromised as well. In one study, scientists at Weill Medical College of Cornell University gave volunteers tiny wounds on their skin by applying and then ripping off pieces of tape, and then exposed the volunteers to a stressful situation: fake job interviews. Their skin took longer than usual to heal. "What's really intriguing is that even relatively mild stress—in this study, the subjects knew the interviews were fake—can affect the way your skin functions," says Richard D. Granstein, MD, chief of dermatology at New York–Presbyterian Hospital/Weill Cornell.

There's also evidence that stress can speed the onset of skin cancer—at least in mice. In a study published in the Journal of the American Academy of Dermatology in December 2004, scientists at Johns Hopkins showed that mice exposed to the scent of fox urine (the rodent equivalent of having a fight with your boss and getting a speeding ticket in the same afternoon) and UV light developed skin cancers in less than half the time it took for nonstressed mice exposed to the light.

Though it can be a factor, inner turmoil is rarely the sole culprit behind misbehaving skin. "You can have all the stress in the world, and you won't get psoriasis if you don't have the genes for it," says Grossbart. "And some people are just physiologically more hardwired to have their emotions trigger skin problems. The flip side is that those people are also more likely to be able to use psychological techniques to improve their skin's condition."
As a reminder, always consult your doctor for medical advice and treatment before starting any program.