There is know hardcore explination for Nocturnal penile tumescence (NPT), or morning wood. The most commonly accepted theory is the nerves that control a man’s ability to have a reflex erection are located in the sacral nerves (S2-S4) of the spinal cord. A full bladder is known to mildly stimulate nerves in the same region. This mild stimulus which during the day is normally suppressed in adult males by competing stimuli and other distractions, could during sleep with the absence of such factors instigate a reflex erection. Thisdoes not occure in men with ED and can happen up to 3 or more times a night.
Several reasons, bladder does fill over night causing circulation restriction, it does get warm in bed causing more circulation, your laying down which causes the blood in your body to travel less of a distance and more centered to your core.
Do you become aroused in the morning?
sometimes... yes... frequently... very often... yes...