Sleeping on your side helps get rid of bad buildup and harmful chemicals in the brain which cause plaques and tangles commonly seen in Alzheimer's disease.

“It is interesting that the lateral sleep position is already the most popular in human and most animals – even in the wild – and it appears that we have adapted the lateral sleep position to most efficiently clear our brain of the metabolic waste products that built up while we are awake,” University of Rochester researcher Maiken Nedergaard said in a release. It's estimated that nearly two in three Americans sleep on their side.
Though further testing needs to be done in humans, the study does support other findings which boost the connection between sleep and Alzheimer's, Parkinson’s and other neurological diseases.
“The study therefore adds further support to the concept that sleep subserves a distinct biological function of sleep and that is to ‘clean up’ the mess that accumulates while we are awake," said Nedergaard. "Many types of dementia are linked to sleep disturbances, including difficulties in falling asleep. It is increasingly acknowledged that these sleep disturbances may accelerate memory loss in Alzheimer’s disease. Our finding brings new insight into this topic by showing it is also important what position you sleep in."
To keep the brain healthy and disease free sleep is essential, say a number of studies. Poor sleep doesn't just affect your mind's ability to save new memories, but it can create a channel allowing the harmful proteins to affect long-term memory storage, says a study from June. Older men who reported sleep troubles were 1.5 times as likely to develop Alzheimer's than their easy-sleeping counterparts, according to study from 2014.
Source-Medindia