When you see a doctor, your doctor will often ask you to do a nocturnal erection test, so what is a nocturnal erection all about? To understand this problem you must first understand the characteristics of penile erections. Medically, erections are divided into three types according to the characteristics based on the formation of erections: ① Psychological erections caused by visual, auditory, olfactory or sexual fantasies are called psychological erections. Psychogenic erection is the erection in the initiation phase, which has a preparatory effect on the subsequent sexual act. Psychogenic erection is affected in people with decreased libido, which means that the hardness of the penis can be problematic in people without ideas. ② Reflex erection is the erection of the penis caused by stimulation of the penis or sexually sensitive areas during sexual foreplay or sexual activity; reflex erection plays a major role in the prolonged maintenance of the erection and hardness of the penis. Decreased penile sensation (e.g., some long-term masturbation, diabetes, age, etc.) can cause reflex erectile dysfunction, and such patients often have no problem with penetration but weaken shortly afterwards. In the case of high spinal cord injury (e.g. thoracic or upper lumbar spine injury), reflex erection still exists or even strengthens, while in the case of low spinal cord (sacral medulla) or pelvic nerve injury, reflex erection disappears. (iii) Nocturnal erection nocturnal erection or penile distension (NPT) in the sleep state, the mechanism is unknown and may be related to the weakened inhibition of the sacral center by the higher center in the sleep state. The average number of nocturnal erections in normal men is 3-5/night. If an impotent patient has normal nocturnal erections, it can at least indicate that his penis itself should be functioning well, and the problem may lie in penile receptor insufficiency, neurological impairment, or psychological abnormalities. Conversely, if the nocturnal erection is abnormal, a cavernous vascular examination should be done.