Why do guys wake up with a hard-on?

It's common for men to have erections during the night -- a phenomenon known as nocturnal penile tumescence (NPT) -- throughout their life. In fact, the penis can actually stay erect longer and be noticeably harder when a guy is asleep than when he's fully aroused and awake.

While we don't know exactly why NPT happens, we do know these erections -- a guy can have three to five a night -- are caused by electrical impulses in the brain that tend to occur during the REM (Rapid Eye Movement) stages of sleep. So, when your dude comes out of the REM cycle when he first wakes up, he may have a still have a raging boner. But it isn't necessarily synonymous with excitement. If he is feeling randy in the morning, chances are it's because of his testosterone level, which can peak in the A.M.