What Causes Semi Hard Erection In The Morning?

I get semi-hard erections in the morning. I know longer get random one through out the day. Some weeks are much better than other and they are easier to get. I am 27 years old and my blood pressure is mild slightly higher than average. It has been this way for several months and use to masturbate frequently but have sense stopped. I have had full erections sparingly though so I don't see it as any possible physical damage. Sometimes when I am attracted to a women I get a shiver like my body it reacting to it. Is it the blood pressure, Mastubation, or just possibly all in my head. I have started excersicing more and trying to eat better as well.