What used to be called neurasthenia (inability to function due to fatigue), now called dysautonomia. Dysautonomia is a disorder of autonomic nervous system (sympathetic-vagal imbalance), which is automatically regulating many important functions in the body such as pulse, blood pressure, temperature and respiration. Controls the response of “fight or start”, as in the case of an attack, the normal response is tachycardia (pulse lifting), blood pressure (BP) and strength, but the answer is Dysautonomia inadequate and bradycardia (low pulse), fall in BP and decreased strength, fatigue and drowsiness.
A good analogy is to consider what happens to the fluid in a bottle half full. If it is placed upward the fluid stays in the lower half of the bottle (the blood stays in the lower extremities). If the bottle is laying down the fluid reaches the cork (the blood reaches the brain).