Before bashing stress, let it be said that experiencing stress is absolutely normal, and it isn't even all bad. Stress is our natural response to a sense of danger. Its evolutionary purpose was to sharpen our senses and reactions so we could be at our best, whether we chose to confront that saber-tooth tiger charging in our direction, or to get out of its way quickly.
"What happens during stress, is that the brain is alerted to the fact that there's some need for the body to respond," explains Dr. Rachel Yehuda, professor of psychiatry at Mt. Sinai School of Medicine. "It activates neural centers in the brain to initiate a variety of reactions, many of them chemical, and they tell the body to do things that people need to do when they need to react. For instance, if you need to run away from a threatening situation, you want to divert energy to the muscles."
Triggered by the brain, chemicals like adrenaline surge through the body during a stress reaction, causing heart rate, blood pressure, and blood sugar to rise in order to get more oxygen and energy more quickly to our muscles. These chemicals also improve alertness, and some research has found that learning and memory are enhanced during a stressful experience.
But turbo-charging the systems you need to take on that tiger right now requires diverting resources from bodily functions you can do without until later. "The body has to cease and desist doing some of the things that it normally does, such as ovulation, digestion. Things like that are expensive and take energy," says Dr. Yehuda, "and then when the stressful situation is no longer present, there needs to be a master shut-down to tell the body that it's okay to get back to normal again."