It seems fairly clear that some positions are just inherently hard to sleep in, no matter how tired you are. I was just wondering whether you could train yourself to sleep in them - even possibly sleep well.
It's a question I occasionally think about, as there are things I can barely sleep during, and I've wondered if I could train myself to do so. Anyone who's followed my posts long-term here will be aware that my interests now lie more in being locked in small containers rather than being chained or bound; but the same principle should still apply all the same.
I have taken to sometimes sleeping in a locked surfboard bag, and I suppose I don't sleep as well as usual there, but I can do so to a degree that seems to suffice. Most bags are too big, so I fill out the interior partially by placing a number of surfboard separating pads in, which not only makes it more snug, but also makes the bag a lot less flexible along its length, which itself significantly limits movement. (Placing the bag inside a slightly larger bag also has a similar effect.) In this situation, I can lie either on my back or my front, but the former is probably better, as the latter requires me to turn my head to one side. It is possible to switch from one position to the other without opening the bag, but it is hard, and tends to make you breathless for a few minutes - so it's not something you want to do very often.
But I have long wondered whether I could train myself to sleep locked in a suitcase, in a curled-up position. I have successfully slept for a couple of hours like that, but not more - and I do have my doubts about whether that is a hard limit I could never get past, no matter how much I practised it. Part of the problem is that, whether asleep or awake, you do tend to ache after a few hours curled up like that, even if the position itself is not uncomfortable. It's not clear to me whether this is inherent, or whether it could be overcome by practising, or maybe increased fitness or flexibility.
So I'm not sure. That's why I was interested to know others' thoughts about it.