Researchers at Stony Brook School of Medicine studied the effect of body posture on the glymphatic system transport in experimental animals. They found that amyloid clearance (good) and glymphatic transport were most efficient in the side-sleeping and back-sleeping position while the stomach-sleeping position (i.e., the most upright position mimicking awake posture) resulted in slower clearance.
According to the authors, the right-sided sleep position was associated with efflux and influx...out through and in through the glymphatic system.
J Neurosci. 2015;35(31):11034-11044