I just flew NZ, UA, and TG in J. The NZ and UA seats were 180 lie-flat, and I slept like a rock. The lie-flat (non-180) on TG was of course WAY better than Y, but I did not sleep quite as well.
As for REM sleep, I also recently flew domestic F on US (barely recliner!), had a dream and an arm spaz, and knocked my seatmate's coke into his lap.

