I read about a study online. it said weekly nightmares can lead to dementia. any truth to this?

A person with dementia may very well have weekly, or even daily nightmares. A healthy person with frequent nightmares may have anxiety, depression or other behavioral issues, but this should not be a progression to dementia. A healthy lifestyle with exercise and good sleep habits are the best protection against nightmares.

Nightmares can be associated with sleep disorders, especially narcolepsy, but also in both normal and emotionally disturbed individuals. But in spite of what you read online, nightmares do not cause dementia. Lack of sleep, however, can cause temporary cognitive compromise which is reversible.