Chen Bo, this is because internal organs such as rectum and bladder are stimulated, thus causing penile reflex erection. When I wake up in the morning, the erection last night has not subsided, or the urine that has been suppressed all night has filled the bladder, and the stimulation caused by the pressure increase in the filled bladder has caused the penis to erect. This is a normal physiological phenomenon, which is called "morning erection" in medicine.
Erection in the morning is a meaningful physiological phenomenon in clinic. Some doctors have specially studied this phenomenon and found that men's erections will weaken or disappear in the morning during their illness; After the body recovers, the erection in the morning will also recover. Therefore, early morning erection can be used as a reference index for men's health.