Nightmares may be an early sign of dementia, scientists say.

The researchers came to this conclusion when they analyzed the results of three studies that looked at sleep quality. The participants in the experiment were followed for several years, assessing the health of their brains. Among them were more than 600 people aged 35 to 64, as well as 2,600 people aged 79 and older. It turned out that those who have nightmares at least once a week in middle age are four times more likely to suffer from cognitive decline in the next ten years. And older participants in the study, who often experienced disturbing dreams, were twice as likely to develop dementia.