For those unsure of terms

lateral = side

supine = back

prone = stomach

I've seen no studies on left side vs right in the glymphatic system - and certainly not in humans.

There have been right-vs left studies in humans, but the only ones possibly related have to do with 'vagal tone', which may have something to do with glymphatic clearance. (TL;DR: left seems a bit better).

If you're wondering how someone might go to sleep in a MRI, there are ones just for your head, permitting a much more comfortable surface. The tube of a full MRI doesn't really allow space for enough padding to sleep on your side (yeah, done that, but not on my side - and the tech was unhappy 'cause he needed me to hold my breath periodically).