Urban Dictionary: A morning erection that is naturally caused as the brains enters the REM-deep sleep phase. At this stage, the body's skeletal muscle structure relaxes and causes hypervasodilation in the capillaries of the body, resulting in said erection.
After sleeping so soundly and having a great night's rest, it is no surprise to wake up with an erection.
Did you know that the proper name for ‘morning wood’ is “nocturnal penile tumescence” and that it normally occurs three to five times per night?
Youtube channel AsapSCIENCE has put together a two-minute video explaining the science behind the mysterious morning raging boner, which you can watch below: