I sleep on my stomach. but most of my flying is seated to supine. I'm going to take a WAG that human G tolerance is better standing, than inverted. I'm talking momentary loads like in a crash with sudden deceleration. The internal organ attachment "cables" are exercised most often, upright,in daily walking about. The opposite constraint structure is not. but that's hypocritical, The question not asked here is neck strain/fatigue. Most seated positions allow normal neck position with head upright. As the seat back angle increases it's more work to keep the head upright to view forward & instruments. By the time you are at sailplane recline postions, a head rest is needed. My supine harness has an adjustable head rest. I've napped in it, hung under a tree while waiting for weather. Prone positions strain the neck too. But worse, since even spine straight look straight down positions require you to hold your head up. ( and if you got prone, you'd better have a window under your head, or why bother? ) Looking forward is more strain. Hang glider pilots build those muscles over time, but even so, after hours of flight it's the neck, not the arms, that aches most. Good hang gliding helmets are cut high in back compared to motorcycle helmets to allow better forward & some upward vision. The design of a head restraint system is more difficult, prone. Since the head restraint must be tied into the harness, not structure. It's the shoulder/neck/head relationship you need to preserve, and without a seat back to index a head restraint, it must be part of the harness. And if you want to crawl into position and not need others to strap you in, you need a harness you wear, ( like a basic training hang glider prone harness ) that you then clip in to your couch? at hips and chest. It may be easier to skip any kind of surface to lie on, and just suspend a full coverage harness from the structure in 3 dimensions. ( the Nausicca replica used this method ) Prone isn't impossible, it just takes at least as much consideration and work as seated. The question is, are the compromises worth the gains?