sleep is sexy
"During REM sleep, your genitals become aroused and you have an erection or increased vaginal lubrication and clitoral engorgement. For example, the common "morning erection" stems from the night's last REM period, often just before waking. Except during very scary dreams, genital arousal always occurs, regardless of whether the dream's content is sexual. The phenomenon has been studied mostly in men, from whom measurements are more easily recorded. In young men, sleep-related erections outlast REM periods, lasting 30 to 45 minutes on average. A typical 25-year old man therefore has an erection during nearly half his night's sleep, a 65-year old man for one-quarter. Many men troubled by "erectile disorder" (impotence) have morning erections, suggesting that the problem is not between their legs."
-Psychology, David Myers