The Effect of Sleep Position on Glymphatic System Function

  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

Restorative Function of Sleep: Yep, the Glymphatic System Implicated

  An important factor affecting glymphatic flow is interstitial space volume which increases 40-60% during sleep.Certain anesthetics that induce Stage 2 sleep and natural sleep significantly increase interstitial space volume.
  Awakening sleeping mice (never a good idea, they're real grouches) dramatically reduces glymphatic flow.
  The authors conclusion: the restorative function of sleep may be a consequence of the enhanced removal of potentially neurotoxic waste products that accumulate in the awake central nervous system by the glymphatic system.
Science 2013;342(6156):373-377