Nightmares after a traumatizing experience might not have anything to do with stress. That's just one surprising finding to come out of what New Scientist calls "one of the largest ever studies of nightmares in the general population." To better understand why some people who've suffered trauma have bad dreams and some don't, Oxford researchers gathered information on 846 people—including alcohol intake, amount of sleep, life struggles, and tendency to worry—then asked them to answer questions about the frequency and severity of their nightmares. They discovered that worrying before sleep boosted the chance of a person having a nightmare. But so did too much sleep, per the study in Social Psychiatry and Psychiatric Epidemiology.
The researchers discovered that occurrence of nightmares increased with more than nine hours of sleep per night, which suggests nightmares aren't simply caused by a negative experience. As study author Stephanie Rek explains, sleeping in long stretches may result in more rapid eye movement (REM) sleep, which is when nightmares usually happen, per the Sun. In fact, the scientists theorize that it may be the disruption of sleep from one nightmare that aggravates worries and causes people to sleep longer on subsequent nights—resulting in even more nightmares. In another surprise, however, the survey showed alcohol, which also results in more REM sleep, was not associated with nightmares. This "warrants more attention," Rek says, per New Scientist. (See how men's and women's nightmares differ.)