A sleep disorder that is little-known and not fully understood could be a sign of developing Parkinson's or dementia. REM sleep behaviour disorder affects around 1% of the general population, but 2% of people aged over 65.
REM is rapid eye movement - one stage of the sleep cycle. It plays an important role in dreaming, memory, emotional processing and healthy brain development.
It's during this stage of sleep that people enter a state of low muscle tone. That means muscles don't 'act out' dreams, reports the Mirror.
