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Nocturnal penile tumescence is a spontaneous erection of the penis during sleep or when waking up. Along with nocturnal clitoral tumescence, it is also known as sleep-related erection.[note 1] All men without physiological erectile dysfunction experience nocturnal penile tumescence, usually three to five times during a period of sleep, typically during rapid eye movement sleep.[1] Nocturnal penile tumescence is believed to contribute to penile health.[2]

Diagnostic valueEdit

The existence and predictability of nocturnal tumescence is used by sexual health practitioners to ascertain whether a given case of erectile dysfunction is psychological or physiological in origin.[1] A patient presenting erectile dysfunction 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 erectile dysfunction 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.[1]


The cause of nocturnal penile tumescence is not known with certainty. Bancroft (2005) hypothesizes that the noradrenergic neurons of the locus ceruleus are inhibitory to penile erection, and that the cessation of their discharge that occurs during rapid eye movement sleep may allow testosterone-related excitatory actions to manifest as nocturnal penile tumescence.[3]

Evidence supporting the possibility that a full bladder can stimulate an erection has existed for some time and is characterized as a 'reflex erection'.[4] The nerves that control a man’s ability to have a reflex erection are located in the sacral nerves (S2-S4) of the spinal cord.[5] A full bladder is known to mildly stimulate nerves in the same region.

The possibility of a full bladder causing an erection, especially during sleep, is perhaps further supported by the beneficial physiological effect of an erection inhibiting urination, thereby helping to avoid nocturnal enuresis[citation needed]. However, given females have a similar phenomenon called nocturnal clitoral tumescence, prevention of nocturnal enuresis (bed-wetting) is not likely a sole supporting cause.[6]

In popular cultureEdit

Research into the causes of nocturnal penile tumescence was the subject of "The Mystery of Morning Wood", a 1995 episode of the animated comedy series Beavis and Butt-head.

See alsoEdit


  1. ^ A morning erection is colloquially known as "morning glory." (Knight, 2016)


  1. ^ a b c "Tests for Erection Problems". WebMD, Inc. Retrieved 2007-03-03.
  2. ^ Why guys rise and, well, rise in the morning?, The Body Odd, NBC News, October 2010
  3. ^ Bancroft, J (2005). "The endocrinology of sexual arousal". Journal of Endocrinology. 186 (3): 411–427. doi:10.1677/joe.1.06233. PMID 16135662. Retrieved September 25, 2013.
  4. ^ "Nervous system control of the male reproductive system". Retrieved 2018-01-09.
  5. ^ Phil Klebine; Linda Lindsey (May 2007). "Sexual Function for Men with Spinal Cord Injury". Spinal Cord Injury Information Network. University of Alabama at Birmingham. Archived from the original on 2013-09-06. Retrieved 2011-12-17.
  6. ^ Scott Beale (Aug 2016). "Why Do Men Get Erections in the Morning". IFL Science. Retrieved 2016-12-03.


  • Knight, Jane (November 2016). The Complete Guide to Fertility Awareness. Routledge. ISBN 978-1138790100.