I do sleep/nap on the side, and put a lot of weight on the ear or the bone in front of it.
I bought quite a few iems in order to find something confortable for sleeping with.
I had no luck with barrel shaped iems, even with small one like HifiMan RE-400 : the pressure make them dig in the ear, and any lateral move changes the sound or break the seal.
I did try the Soundmagic PL50, and it is indeed the most confortable of all : very small, flat, keeps the seal. Cable is so-so. And i'm one of those who dont like its sound :) It sounds nice, it is bright, it's good with classical and jazz. But it has some heavy narrow peaks. When trying to EQ it flat, i had to use : -13dB at 2330Hz Q=24/0.06oct , -8dB at 6550Hz Q=8/0.18oct, -6dB at 8500Hz Q=13/0.11oct. And it needs some bass boost. As I like to listen to ambient electronica to drown out neighbours noise, I need some bass to make the sound "whole", and any drone in some frequencies (eq'ed down above) would hurt.
I spent quite some time trying to live with this PL50, but no, not for me.
I tried a few other flat IEMs : Meelectronics M6 (flat, but body a bit too wide), some budget JVC (not flat enough).
But my happiest finding was the Jays T-Jays. I even bought each of the serie, in that order : T-Jays Three, T-Jays One, T-Jays Two.
General comment about the family, and why I like them :
They sound good and balanced. They all have good bass. The first days, with each of them, I thought the bass was too strong, but after a few days, I dont think that way anymore. The bass is there for my ambient, not overbearing, and the sound is good.
I can't wear them straight down as they are supposed to be worn : their big and sharp body hurts my ear. May be I just have small ears.
But cable up, they are the most confortable I had, and i can sleep on them for hours. Only downside is that channels are inverted (left on right, right on left). So, you have to not be bothered by that, or have a player that can swap channels (there is a patch for that for rockbox that one can custom build).
Also, their cables are quite small : 60 cm. Which I like ! I can just clip a sansa clip+ or clipzip on my chest, and sleep and change sides without messing with a long cable.
For an even better confort, I use them with comply T-200, inverted. Inverted, they sound better, and u can adjust the position so that the pillow pressure is distributed between the iem body and the tips' tip.
I'm so happy with them that I dont use better sounding IEMs anymore (re400, philips fidelio s1), even when not sleeping : the small cable and their confort are such a big plus.
About the differences in the family : Two is balanced, not offending, sounds good. One is similar to Two, with a bit less quality in the sound, a bit less clear. Three has some upper bass/lower mids enphasis than makes them a bit muddy with classical. I prefer the Two.