The reasons why the penis keeps getting hard after sleeping include sleep disorder, prostatitis, age factor and so on. 1. Sleep disorders: Normally, men experience several penile erections during the night, but they usually don’t feel them due to sound sleep. If the sleep is light, then you may wake up and feel the erection when the penis is erect. 2. Prostatitis: If you suffer from prostatitis for a long time, it may cause stimulation to the penis and sexual nerves, so that the erection center is always in a state of excitement, showing the phenomenon of continuous hardening. 3. Age factor: as men grow older, the vegetative nerve function may become abnormal, and the problem of frequent penile erection at night may also occur. There are many reasons why the penis stays erect after sleeping, if the erection symptoms persist for a long time, you should go to the hospital for examination and treatment according to the doctor’s advice.