We usually think nightmares are problems that interrupt the sleep of children. But half to 85 percent of adults have nightmares too. About 1 in 20 adults report experiencing them every week. In a new study published in the journal eClinicalMedicine, U.K. researchers observed that adults between 35 and 64 years old who experienced distressing dreams were four times more likely to experience cognitive decline. Men with nightmares and distressing dreams were found to have a higher risk of later cognitive decline and dementia than women. While the study doesn’t show exactly why nightmares can so negatively impact the brain, the findings make sense because these dreams affect sleep and rest, both of which have been shown to affect cognitive decline and dementia.