A hard penis waking up from sleep can be physiological, or it can be due to an infection of the urinary system, or a spinal cord injury. 1. Physiological phenomenon: men enter puberty, the body androgen secretion is strong, with the secretion of hormones, there may be a sleep erection resulting in the penis waking up hard symptoms, usually found in the morning erection belongs to the morning erection is a normal physiological phenomenon, do not need treatment. 2. urinary tract infections: such as prostatitis, seminal vesiculitis and orchitis, epididymitis and other diseases, may also lead to sleep erection caused by the penis waking up hard symptoms. 3. Spinal cord injuries: spinal cord injuries, such as trauma, may affect normal physiological functions, leading to symptoms of hard penis. To summarize, if it is a physiological phenomenon need not be treated, if it is a disease factor, you should actively seek medical treatment, follow the doctor’s instructions.