Association of nocturnal erections with REM sleep

Penile tumescence during the different stages of sleep. Nocturnal erections occurred during periods of REM sleep (columns).

REM: rapid eye movement.

Data from: Karacan I, Williams RL, Thornby JI, Salis PJ. Sleep-related penile tumescence as a function of age. Am J Psychiatry 1975; 132:932.

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