Nightmares are bad enough, but this news makes them even scarier … A new study says they may be a warning sign that you’ll develop dementia later in life. In the study, people between the ages of 35 and 64 who had bad dreams at least once a week were four times more likely to experience cognitive decline in the following decade. And it was much more prevalent in men -- they were five times more likely, vs. just a 41% greater likelihood in women.