Morning wood

Morning wood is an erection on awakening. It's more often called a piss-hard.

Males often become erect during sleep, for reasons which are reasonably 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. Most often a full bladder creates morning wood.

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