Nocturnal penile tumescence

In anatomy and physiology, nocturnal penile tumescence is the spontaneous occurrence of a penile erection during sleep in the absence of any sexual stimulation. All men who are physically able to achieve erection do so during sleep, typically several times in a night^[1]. It typically happens during REM sleep and it is not uncommon for an erection to be present when a man wakes up — such an erection is colloquially referred to as morning wood in North America, and as morning glory in the United Kingdom (though in some areas of the U.K. the adjective piss-proud is often used instead).

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The existence and predictability of nocturnal tumescence is used by sexual health practitioners to ascertain whether a given case of erectile dysfunction (E.D.) is psychological or physiological in origin. A patient presenting with E.D. is fitted with an elastic device to wear around his penis during sleep; the device detects changes in girth and relays the information to a computer for later analysis. If nocturnal tumescence is detected, then the E.D. is presumed to be due to a psychosomatic illness such as sexual anxiety; if not, then it is presumed to be due to a physiological cause.