For me, podcasts specifically about going to sleep to them trigger my contrariness too much to be actually relaxing. It's gotta be on a normal topic that is just the right balance of interesting, but not exiting/engaging.

History typically scratches that itch for me. Dan Carlin's hardcore history and the history of the English being the two goats that coke immediately to mind. Camp Monsters is also a great one; the rare fiction podcast that I can actually stand, much less relaxes me enough to sleep.