"Fear makes men forget, and skill that cannot fight is useless."The sympathetic nervous system mobilizes the body's energy reserves during times of stress. It neutralizes processes controlled by the parasympathetic nervous system, such as digestion, while ramping up secretion of adrenaline and noradrenaline, dilating bronchial tubes in the lungs, tensing muscles, and dilating heart vessels.
It also causes your heart rate to increase.
There's a direct relation between stress-induced heart rate and both mental and physical performance. Too low, such as when you're just waking up, and you can't think or react very quickly. Too high, and one's ability to think and perform motor skills degrades.