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.)