Morning erections are a normal part of male physiology.
In fact, when doctors are trying to determine whether or not a man's erectile dysfunction is physiological or psychological, they will often apply a nocturnal tumescence monitor, which monitors the presence and duration of erections during the night.
Several things can cause morning erections.
One, as the bladder fills during the night, it tends to trigger a spinal reflex that can cause erection. Also, the congestion caused by the full bladder can decrease the outflow of blood from the penis.
There are aspects of normal sleep cycling that tend to induce erection as well.