Males sometimes become erect during sleep, for reasons which are reasonably well understood. These include:
- Erotic dreams, with or without nocturnal coming;
- Stimulation from friction of bedclothing;
- Periodic autonomic system arousal;
- Having a full bladder, which can create pressure on the prostate.
In a D/s context, a submissive may be required to cause an erection during the night or as a means of waking up the dominant male, so as to provide an opportunity for his use. For example, the dominant may choose to both piss and come on his submissive, perhaps in their mouth.