OK. For the ever-curious SS4, here’s a quickie rundown of how it works.
The sympathetic and the para-sympathetic nervous systems are at conflict, at least when it comes to erections. Whereas the sympathetic nervous system - reacting to negative influences like cold, stress, anxiety, guilt, and a lot of other issues – tries constantly to prevent erections, the para-sympathetic system encourages them.
While we may not always like the result [new, delicious and hot girlfriend/boyfriend who makes us so nervous we can’t get a hard-on], there are probably some very sensible reasons why we evolved this way.
To make a broad generalization, it is the sympathetic nervous system that often keeps us safe from harm by our environment. And, without the sympathetic we’d be having a whole lot of inappropriate erections. Cave men, for example, could not go running around in search of food in the forest with an erection without damaging it on a tree limb or a rock. If you were giving a business presentation in front of a group of clients, you wouldn’t want your dick acting as the pointer for your PowerPoint demo. When you are “safe,” though, with a partner who makes you comfortable and turns you on, the para-sympathetic system overcomes the sympathetic and you get an erection – a “safe” one.
During REM sleep, however, the para-smypathetic system takes over. The sympathetic neurons in the locus coeruleus are turned off and erections are allowed to occur spontaneously, no interference at all from the sympathetic system and so a completely open and different route taken for the erections to occur.
Without the inhibitions of the sympathetic system operating, it makes sense that nocturnal erections, unhampered by it, could reach their full potential for size and be larger often than sexual ones we experience while we are conscious with both (conflicting) systems operating.
How you can control this during conscious hours should have a thread of its own.