“The more one is occupied with the solution of dreams, the more willing one must become to acknowledge that the majority of the dreams of adults treat of sexual material and give expression to erotic wishes," wrote Sigmund Freud in the first line of his famous Sex in Dreams. In other words, dreams and sex are inextricably intertwined. 
 
 According to data collected by the Kinsey Institute, 37 percent of women aged 45 have experienced an orgasm caused by a sex dream; five percent reached orgasm for the first time after having a dream. As for men, 83 percent have experienced a nocturnal emission; whether or not it was prompted by a dream was not specified. Erotic dreams are abundant in the REM worlds of both genders, with 70 percent of women and 100 percent of men having overtly sexual dreams. 
 
 But is there a way to induce an x-rated experience? Not exactly. But you can increase your chances. The journal Dreaming published a study that found those who slept on their stomachs with their arms raised above their heads were more likely to have x-rated nighttime episodes. 
 
 For the study, 670 students were asked to self-report their dreams and sleeping position. Those who frequently slept on their stomachs were more likely to experience dreams relating to “sex,” “being tied up,” “being unable to move” and “being locked up.” The most common theme was “erotomania,” also known as De Clérambault's Syndrome — the belief you have a wealthy, often famous, secret admirer. 
 
 What makes those who lay prone so, well, prone, to such dreams? Researchers believe that the position puts pressure on the genitals and that external stimulus gets remixed into our internal movies. 
 
 "I believe that the brain during sleep is not at all totally detached from the external world, and stimuli, including those stemming from the environment, are probably incorporated into dream content more often than people observe or are aware of," said lead researcher Kai-Ching Yu when the study was first released. "The unconscious brains of the dreamers try to make sense, and even make use of, the external stimuli." 
 
 If you want to increase the chances of nocturnal trysts, however, know that sleeping on your stomach is actually one of the worst ways to conk out, as it compresses the spine leads to lower back pain. Worth it? You decide. ||||| She invited 11 volunteers to spend a few nights at her laboratory. They slept in a hulking medical scanner that measured their brain activity, while electrodes on their heads and hands measured their brain waves, eye movements, heart rate, and more. “The scanner has a bed that could go completely flat, and we put a lot of pillows and towels to make it comfortable,” says Sasaki. “It was a little restricted but people could still sleep.” And sure enough, they took longer to fall asleep and slept less deeply on the first night. 
 
 While they snoozed, team members Masako Tamaki and Ji Won Bang measured their slow-wave activity—a slow and synchronous pulsing of neurons that’s associated with deep sleep. They found that this slow activity was significantly weaker in the left half of the volunteers’ brains, but only on their first night. And the stronger this asymmetry, the longer the volunteers took to fall asleep. 
 
 The team didn’t find this slow-wave asymmetry over the entire left hemisphere. It wasn’t noticeable in regions involved in vision, movement, or attention. Instead, it only affected the default mode network—a group of brain regions that’s associated with spontaneous unfocused mental activity, like daydreaming or mind-wandering. These results fit with the idea of the first-night brain as a night watchman, in which the left default mode network is more responsive than usual. 
 
 To test this idea, Sasaki asked more volunteers to sleep in a normal bed with a pair of headphones. Throughout the sessions, the team piped small beeps into one ear or the other, either steadily or infrequently. They found that the participants’ left hemispheres (but not the right) were more responsive to the infrequent beeps (but not the steady ones) on the first night (but not the second). The recruits were also better and quicker at waking up in response to the beeps, when the sounds were processed by their left hemispheres. 
 
 This shows how dynamic sleep can be, and how attuned it is to the environment. The same applies to many animals. In 1999, Niels Rattenborg from the Max Planck Institute for Ornithology found that ducks at the edge of a flock sleep more asymmetrically than those in the safer center. “In this way, sleeping ducks avoid becoming sitting ducks,” he says. Fur seals do something similar; they sleep in the usual way on land, but at sea, they sleep on one side with the open eye looking down, perhaps to watch for sharks. 
 
 “It’s very exciting to see that researchers have now found something similar in humans,” says Rattenborg. “It seems reasonable to speculate that, as in ducks and seals, this is an adaptive response that provides us with some protection when sleeping in novel environments, wherein we have limited information about potential threats.” 
 
 Lino Nobili from Niguarda Hospital in Milan adds that these results fits with a “relatively new view of sleep” as a patchwork process, rather than a global one that involves the whole brain. Recent studies suggest that some parts can sleep more deeply than others, or even temporarily wake up. This might explain not only the first-night effect but also other weird phenomena like sleepwalking or paradoxical insomnia, where people think they’re getting much less sleep than they actually are.
– Matt Berical noticed that his restless nights always ended with him facing right, while his girlfriend mostly reported nightmares after waking on her left. "Is there, I wondered, a correlation between bad dreams and sleeping position?" writes Berical at Van Winkles. He dug up a 2004 study that asked 45 men and 18 women to sleep on one side or the other and fill out a popular sleep questionnaire called the Pittsburgh Sleep Quality Index (which evaluates sleep using seven factors, including "subjective sleep quality," sleep duration, and use of sleep drugs). The study found that just 14.6% of right-side sleepers reported bad dreams, compared to 40.9% of their lefty counterparts. Right-side sleepers also reported more dreams with feelings of safety or relief, but had worse-quality sleep than left-siders. Berical also found a 2012 study that analyzed the sleep positions and dreams of over 670 students. The study found that face-down sleepers had more positive and vivid dreams, which were more often about "sex," "being unable to move," "being tied up," or "being locked up," Van Winkles reports. The researcher behind that study, Calvin Kai-Ching Yu, concluded that "the brain during sleep is not at all totally detached from the external world," adding that "the unconscious brains of the dreamers try to make sense, and even make use of, the external stimuli." Berical's conclusion? "It can’t hurt to roll your loved one over when you sense a nightmare coming on," he writes. "Assuming you can avoid the flailing arms, that is." (It turns out that humans are incredibly good at sleeping.)