CBS Local — Excessive sleep is linked to more nightmares, according to a new study. The study found that more than nine hours of sleep can result in an uptick in nightmares.

Researchers surveyed 846 people about sleep quality, nightmares, worries, drinking and exercise. They found that worrying about the future before bed was a strong cause of nightmares, which made sense to them. But researchers were surprised to see that excessive sleep also was linked to causing nightmares.

“For example, worry can be effectively treated using cognitive behavioral approaches,” said lead author Stephanie Rek via New Scientist. “It would be interesting to do more research to see whether these alleviate nightmares.”

Surprisingly, neither alcohol nor exercise had an impact, negative or positive, on nightmares. Researchers hypothesize that longer sleep increases rapid eye movement (REM) sleep, leading to more nightmares as most of them tend to happen during REM sleep.

“A greater length of sleep might increase the amount of late-night REM sleep, thereby increasing the opportunity for nightmares,” the study read. “In contrast, insufficient amount of sleep might be related to physiological hyperarousal, lowering the threshold for awakening.”