[quote]Evidence supporting the possibility that a full bladder can stimulate an erection has existed for some time and is characterized as a 'reflex erection'. 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.[4] A full bladder is known to mildly stimulate nerves in the same region.

The possibility of a full bladder causing an erection, especially during sleep, is perhaps further supported by the beneficial physiological effect of an erection inhibiting urination, thereby helping to avoid nocturnal enuresis.[[/quote]