Feeling tired after having sex may be caused by normal physical exertion, or it may be caused by excessive indulgence and sympathetic relaxation. 1. Physical exertion: when having sex, it is necessary to consume a certain amount of physical energy, heat, especially action is too violent, or too excited to accelerate the circulation of blood, which will consume more physical energy and heat, so there will be the phenomenon of feeling very tired after sex. 2. Overindulgence: If the number and frequency of sexual life is too much, it will lead to physical exhaustion, especially men will be more obvious, so there will be after the end of the situation of tiredness. 3. Sympathetic relaxation: mainly men will have this situation, is a physiological reaction after the end of sexual life, is a normal phenomenon, due to the reflex function of the cerebrospinal cord to control the discharge of ejaculation, after ejaculation will soon enter a state of relaxation, which is easy to cause a sense of fatigue. It is recommended to control the frequency of sexual life appropriately, if the problem persists, it is recommended to go to the hospital in time.