Sleep position—whether you sleep on your back, belly, or side—can affect what you dream about, says study author Calvin Kai-Ching Yu, PhD, an associate professor at Hong Kong Shue Yan University. Dr. Yu tested nearly 700 adults and found that one sleep posture in particular is the key to the weirdest, scariest, and sexiest dreams: Belly-down. MORE: What Do Your Dreams Mean? Dr. Yu found that prone sleeping was strongly associated with all kinds of charged dreams, such as having sex, being locked up, being smothered or unable to breathe, having a love affair with a celebrity, seeing a UFO, and being naked. How come? The physical nature of sleeping prone—being pressed against a bed—could induce sexual dreams, says Dr. Yu. (So what does that dream about your ex mean? See Dream Emotions Interpreted.) But prone sleepers—and their spicy dreams—are the exception rather than the rule. Most people sleep in several different postures, but lateral position (on one’s side) is by far the most popular, followed by back, and then belly, says Dr. Yu.  So what do non-belly-sleepers dream about? A small 2004 study in Sleep and Hypnosis, found that left-side sleepers had significantly more nightmares than right-side sleepers, whose dreams evoked more safety and relief. While the jury’s out on why this is exactly, traditional Chinese medicine has a theory: The right side of the body is believed to have masculine, grounding, and physical energy, whereas the left side is seen as having intuitive, emotional, and feminine energy. Being injured on one side is said to cause an imbalance of either your male or female energy—so it follows that sleeping on your left side would mean more upsetting dreams. Regardless of whether you tend toward flying saucers beaming you up with Mr. Clooney, or you’re more of a cuddled-up-in-a-giant-cocoon kind of gal, there’s no question that you need a good night’s sleep. Check out 20 Ways To Sleep Better Every Night.