Bothered by nightmares? You might be spending too long under the covers. People who sleep longer each night are more likely to have scary and disturbing dreams, new research from the University of Oxford discovered.

In the study, researchers surveyed 846 participants about nightmare occurrence and severity, as well as other factors like how long they slept each night, how much they stressed, and other mental factors. They discovered that people who slept longer each night were 35 percent more likely to experience nightmares than those who slept shorter durations.

The study didn’t give specific ranges, but mentioned that past research has linked sleep duration of 9+ hours a night to more frequent nightmares than sleeping between seven and nine hours nightly.

Other factors that were linked to nightmares? Higher levels of worry, depersonalization—feeling disconnected or detached from yourself—and paranoia.

Not terribly surprising: The same negative emotions and anxieties that run through your brain when you’re awake may also run through your brain while you’re sleeping, possibly contributing to scary dreams, as we reported in the past.

But the fact that sleeping too long can play a role is surprising—especially since short sleep is usually what’s linked to health problems. The researchers believe it may be because longer sleep allows for more late-night REM sleep, which increases the opportunity for nightmares to occur.

Good news is, it might be possible that narrowing your sleep window can help you cut down on bothersome nightmares, the researchers say—experts recommend between 7 to 9 hours a night. Might be a good habit to get into anyway, since sleeping too long is actually linked to its own host of health risks.

source: Menshealth