I personally find that exercise only contributes to good mental health outcomes when it is part of a system that monitors and directly, daily, addresses issues of physical exhaustion. Experiencing exhaustion and a desire to push through it was my personal #1 link to depression and anxiety. I found that just laying in bed and relaxing has a much higher likelihood of vanquishing my depression and anxiety than exercise…
Interesting. Lying in bed is pretty much my #1 link for depression and anxiety (unless I'm sleep deprived and need to sleep).
However I'd say that "laying down because it's all I can do" and "laying down as a conscious way of attending to my personal exhaustion" are two different states of being, with an important difference. The first could easily raise one's anxiety if it seems counter to "what needs to be done" while the second approach is part of a more analytical process (at least in my case) which is hopefully accompanied by fail-overs and follow-ons, like addressing stressors, mentioned above.