I Am 21. I Am Very Embarrassed. I Have A

I am 21. I am very embarrassed. I have a very bad night penile erection problem. My penis become so hard and very annoying for hours and hours at night without any reason. It don’t cares if I am sleeping or not. It happens if I am lying in sleepy mood. I don’t work physically much. I feel that I am a matured man, I have control in my mind, I don’t think of erotic stuffs. What is the solution - exercise, changing lifestyle ….. or what and how? Help please.