For people with a certain sleep disorder, a simple blood test could help predict the development of dementia years before symptoms appear, a new study indicates. Idiopathic REM sleep behavior disorder (iRBD) causes people to physically act out their dreams while sleeping. The disorder is also associated with a very high risk of Parkinson’s disease and a related condition called Dementia with Lewy Bodies. This is a form of dementia that often causes memory and cognitive loss, as well as vivid visual hallucinations and movement difficulties similar to Parkinson’s.