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For those with penises, you may be surprised to learn how frequently your member stands up while the rest of your body is rendered catatonic by the muscular paralysis that keeps you from acting out your dreams. (And thank goodness for that. Carlos Schenck and his colleagues [pdf] from the University of Minnesota Regional Sleep Disorders Center describe the case of a 19-year-old with sleep-related dissociative disorder crawling around his house on all fours, growling, and chewing on a piece of bacon—he was ‘dreaming’ of being a jungle cat and pouncing on a slab of raw meat held by a female zookeeper.) Scientists have determined that the average 13- to 79-year-old penis is erect for about 90 minutes each night, or 20 percent of overall sleep time. With your brain cycling between the four sleep stages, your "sleep-related erections" appear at 85-minute intervals lasting, on average, 25 minutes. (It’s true; they used a stopwatch.) I didn’t come upon any evolutionary theories or a proposed "adaptive function" of NPT, but we do know that it’s not related to daytime sexual activity, it declines (no pun intended) with age, and it’s correlated positively with testosterone levels. Females similarly exhibit vaginal lubrication during their REM-sleep, presumably with many dreaming of erect penises.

[NSFW:5f565db485]Source: Has nude artwork: http://www.scientificamerican.com/blog/post.cfm?id=sex-sleep-and-the-law-when-nocturna-2011-05-20[/NSFW:5f565db485]

Women always wonder why when traveling guys refuse to share beds and would rather just sleep apart on the floor or whatever, but anything to avoid being in the same bed. Does this answer it?