My type of OI is NMH, not POTS. (Hmm, do I win a prize for the highest ratio of abbreviations to words?
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(for the newcomers, OI = Orthostatic Intolerance, NMH = Neurally Mediated Hypotension, POTS = Postural Orthostatic Tachycardia Syndrome)
But I think my experience might still be relevant since the issue of sitting upright vs. reclining is still the same.
I think it's very hard to predict how long you would have to rest to recover from time sitting upright. Back when I was still working there were days when I could sit at a desk for several hours without feeling problems. Then there were other days when I'd start to feel wiped out and get waves of nausea after only 20 minutes (I'd put my head on my desk and wait for it to pass). There are so many other variables going on (how well you slept, time of the month, etc.) that I think you just have to play it by ear.
The best solution would be to find a way to work on the computer that does not wear you out as much. Ever since I got a laptop, for example, I can work on the computer longer. I sit in the recliner with my legs up, leaning slightly back, with the laptop in my lap (where else?). I still have to be careful that I don't get too worn out. I take breaks where I rest my arms, lean my head back (neck gets tired), and so on. But this position is much better than sitting in a normal chair at a desktop computer.
You mentioned potential back problems from being in a recliner. One suggestion is to do a few very minor stretches and strengthening exercises for the back. One exercise I like is to lie flat on the floor with your knees bent and then push your lower back to the floor and hold it. You only need to do a couple of repetitions of that one.
Another suggestion is to take breaks. Get up and move around every couple of hours. I end up doing this without even trying because I break up tasks into smaller bits - put a few dishes in the dishwasher, lie down in the recliner, get up and start a load of laundry, lie down in the recliner, etc.
I've been resting in the recliner, for many hours a day, for at least 10 years. And yet I don't have low back problems. I don't know whether I'm just lucky (good genes? stronger core muscles?) or there's some other factor that has kept me from having any back problems. I'm just happy that it works for me.
I wonder whether the connection between recliners and back problems only applies to certain people? Do you have a history of low back problems and that's why you mentioned it?