Nocturnal penile tumescence is the medical name for a spontaneous erection that occurs during sleep or upon waking. Unlike typical erections, these spontaneous erections are not stimulated by sexual thoughts or activity.
Erections occur as early as infancy or even in utero and may persist for as long as a man has an ordinary erectile function. Nocturnal penile tumescence has also been observed in a variety of mammals. A similar occurrence has been noted in women where the clitoris and vagina swell spontaneously.
Nocturnal penile tumescence is sometimes abbreviated to the acronym NPT. The phenomenon is also known by the colloquial terms morning wood or morning glory.