Waking up in the morning with an erection is common and completely normal. However, scientists still have trouble explaining this phenomenon. Most theories are based on the idea that morning erections are caused by a change in the nervous system during REM sleep, as the parasympathetic nervous system takes over the sympathetic nervous system. This change blocks the production of a hormone that inhibits penile erection. It also releases testosterone production while relaxing the muscles, allowing blood to flow to the penis and causing erections during the night.