In the days of the caveman, stress often came in the form of physical threats that required individuals to react quickly and decisively. The body helped out by automatically clicking into high gear at the first sign of trouble, releasing a surge of hormones (notably adrenaline and cortisol) to accelerate the heart rate, raise blood pressure, increase blood sugar, and enhance the brain’s use of glucose. This stress response meant the caveman was instantly ready to fight or flee.