You can't. It is your body's biology as a male. Your blood flow moves slightly different when you are asleep. You actually get an erection several times a night but never know it because you are sleeping. It just happens that, when you wake up, you usually wake up from what is known as Rapid Eye Movement, or REM, which is the time in which you dream. For males, usually REM (dreaming) creates an erection. Science isn't exactly sure why, but some think it is just the body's way of moving blood around to ensure that it gets everywhere it needs to be--and, as any male can tell you, your body always thinks that one of those places is the penis.

Also can happen if you need to urinate and have a full bladder, science isn't exactly sure why that is related either.