NEW YORK -- About a third of adults say they don't usually get enough sleep. Now, there's news of a link between the quality of sleep and the risk of dementia.

A new study takes a look at rapid eye movement sleep (REM), or the period when a person dreams.

There are four stages in the sleep cycle. In stage one, sleep is very light, and it progresses to deep sleep in stage three. After that, there's REM sleep, which usually occurs four to five times over eight hours as your sleep cycle repeats.

In the study, as the amount of REM sleep declined, the risk of dementia increased. The study looked at 321 people over age 60.

There is increasing evidence that toxins accumulate in the brain during the day, but are cleaned out when we're asleep -- sort of like a garbage collecting function -- and these include things like amyloid, which is linked to Alzheimer's.

It's not clear exactly when during the sleep cycle it occurs, but it's a very exciting new area of research.

Research continues, however, on whether sleep deprivation can actually cause dementia. But, we do know that changes in the brains of people with Alzheimer's begin decades before symptoms are noticed.

So at the very least, testing people for sleep problems might be a useful screening test for picking up dementia sooner -- rather than later.