This can happen for two reasons: involuntary erection during sleep and a full bladder. Every man has four or five involuntary erections during the night (when he’s sleeping), but as they come and go during the heaviest moments of his nap, he usually doesn’t even notice. “It’s a penile self-defense mechanism to prevent fibrosis of the cavernous bodies”, says urologist Eduardo Bertero, from the Brazilian Society of Urology.
In other words, to prevent the bilau from losing its function due to lack of use, the body provides involuntary erections. This happens during the period of heaviest sleep, called REM sleep (acronym for rapid eye movement), when the nerves of the sympathetic nervous system (which normally inhibit erection) are superimposed by those of the parasympathetic nervous system (which stimulate erection) – it is as if the former were turned off and the stimulators acted alone.
This mechanism is fundamental for diagnosing the severity of erectile problems: if men with erectile dysfunctions have these nocturnal erections, doctors conclude that the problem is purely psychological. In the case of a full bladder, the erection happens for a mechanical reason. The swollen bladder puts pressure on the erect nerves, which are responsible for transmitting the nerve impulses that command an erection.