Morning wood

Males often become erect during sleep, for reasons which are not well understood. Erotic dreams, with or without nocturnal coming, are clearly not the only reason. Stimulation from friction of bedclothing, periodic autonomic system arousal during sleep, etc have all been suggested as causes.

In a D/s context, a submissive may be required to deliberately cause an erection during the night so as to provide an opportunity for the Master's use.

However it may come to be, an erection on awakening is often called morning wood.