[vram22]

>It's almost as if it's lack of constant (even low-grade) activity that's the problem, rather than whether you're sitting or standing.

Yes, IMO. And I would add to that: it is likely that the lack of a change is also part of the problem, as in lack of change (at not-too-long intervals) between action and non-action (such as walking or moving for the former, and standing, sitting or lying down for the latter). Noticed it in myself - if I walk or move for too long (many hours), or stand or sit or lie down for too long (ditto, except for sleep), I start feeling uneasy or affected in some way (obvious way is to get tired due to the former, but also uneasy due to the latter).

Edited for typo.

Also, adding to above my points in this comment: about lying down for too long, in particular, I've noticed that if I do that, whether intentionally (not often, maybe sometimes due to getting engrossed in reading something interesting), or accidentally / unintentionally (say due to a fever), the body feels uncomfortable after a while, which seems to reinforce my point above about some change being good.