Doing cognitive task horizontally is less prone in giving me symptoms compare to sitting or standing.
And within my cognitive tasks, I have developed strategies of switching between tasks to make sure I don't tire too quickly while doing a task requiring too much concentration. So on the ipad, I may be preparing a letter, then I will quit that for a bit and go check facebook for a little while, then I will go check You Tube. then I returnto my letter. Then to relax my brain, I will check out beautiful things on pinterest. And to get away from my brain, I then start a handcraft in order to do something mindless with my hands and relax the brain.
Like I said, lots of switching of activities, so my brain still functions, but differently, with less concentration required, and of course the option to drop everything and have a nap or a rest for a few minutes (or more)
I hope this helps.