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Nocturnal penile tumescence is the medical name for a spontaneous erection which occurs during sleep or upon waking. Unlike typical erections, these spontaneous erections are not stimulated by sexual thoughts or activity.
These erections occur as early as infancy, or even in utero, and may persist for as long as a man has 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.
Nocturnal penile tumescence is very common. Most men with ordinary erectile function experience it between three and five times a night. Each erection may last for 30 minutes or even longer. Nocturnal penile tumescence is most common during the rapid eye movement (REM) sleep phase.
Health professionals are uncertain why nocturnal penile tumescence occurs, although there is some evidence to support the theory that a full bladder may cause this type of erection. Other theories suggest that noradrenergic cells which inhibit the penile response are turned off during sleep, or that NPT is triggered by the release of nitric oxide in the body while at rest.
Nocturnal penile tumescence is thought to contribute to the health of the penis by oxygenating the penile tissue. Sexual health experts have studied NPT, tracking when it occurs, to determine whether erectile dysfunction may stem from psychological or physiological problems.