The hardening of the penis in the morning, i.e. morning erection, can be related to the cyclical changes in the secretion of sex hormones, holding urine, stimulation and other factors. 1. Cyclic changes in sex hormone secretion: male androgen secretion is cyclic and can reach its peak in the morning, so it can cause morning erections. 2. Urine holding: During the night sleep, rarely urinate, so in the morning, the bladder will accumulate a lot of urine, which can compress the genital veins and impede the return of blood to cause morning erections. 3. Mechanical stimulation: If a man is covered with a thick blanket or wears underwear with a rough texture that continuously stimulates the glans, it may also cause the genitals to become engorged with blood and cause morning erections. Morning erection itself is usually a normal physiological reaction, but it can also be seen in some diseases, if you find morning erection abnormalities, it is recommended to consult a regular hospital in a timely manner.