Plus, a sex expert weighs in on what causes them.

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It started when I was in high school, before I even knew what an orgasm was.That’s right—the first time I experienced an orgasm, I was sleeping. In the middle of a dream that had absolutely nothing to do with sex, I felt a down-there tickle.I almost woke up, and then I felt something that I couldn't explain.It swelled and surged like a current through my body. Then, all at once, it was like I was a puppet and my strings were pulled so tight I shot up out of bed.

At the time, I didn't know what had happened—I just knew it was fantastic. But I later learned that I had experienced a sleep orgasm (or "nocturnal orgasm," to use the technical term). According to sex and relationships expert Ian Kerner, PhD, there hasn't been a lot of research on nocturnal orgasms, but many of his patients report experiencing them. "During sleep, the body is relaxed, and in REM phase sleep, there can be increased blood flow," he explains. "I would say [nocturnal orgasms] are a combination of being relaxed, increased blood flow to the genitals, and the possibility of sexy dreams that would increase psychogenic arousal."

For my part, I experience sleep orgasms about twice a month, sometimes more or less depending on how stressed I am. My husband, who is always in bed with me, knows when they happen—and the dream often results in a steamy real-life romp after I wake up.

Let me guess: You're probably wondering how you can orgasm in your sleep. But unfortunately, even after 20 years of experiencing sleep orgasms, I can't necessarily control whether or not I have one. What does seem to help, though, is going to bed feeling super-relaxed. If I'm not tense, I'm much more likely to feel that slight sensation that indicates something is on the way. Once that happens, if I'm lucid enough, I instruct my brain to go with what's happening. (Not sure how to do that? Think about all those times you've talked your way out of a nightmare by telling yourself, This isn't real—you can use the same tactic to reach the big O while dreaming.)

When I feel that unmistakable sensation, my mind clicks, and I try to work within my dream to help it spark like a firework instead of fizzling out. If you are having a sex dream, I find that you can "script" it to be even hotter. And most importantly, if you have the impulse to wake up, don't—try your best to stay in the moment. In my experience, having a sleep orgasm is usually as easy as just not fighting it.