Not to my surprise, I've also discovered that pain makes me tired. Certain types of pain, especially so, like those sore backs or aching shoulder muscles. I've done a number of things to try and keep myself healthy (which occasionally cause those aching muscles), but sometimes the pain just comes in and wears on me.
Key in that is wearing on me... wearing the body down. Pain can wear someone's physical energy down as the body fights to right what's wrong, but it can also wear someone's mental energy away as they fight to ignore the pain, to struggle on with what they feel they should be doing rather than just concentrating on the pain and how to manage it.
I've not made the direct connection like this before, but this means if I want to do well at writing, or at least if I want to continue to consider it a hobby and not a chore, I should try to make sure I'm as well-rested and pain-free as I can be. Some of this might involve changing when I do my writing -- mornings perhaps instead of evenings -- but a lot of it has to do with two things.
First, I need to pay more attention to those small pains when they are small, and at a minimum do something to keep them from growing if I can't eliminate them entirely.
Second, I need to stop my bad spiral where I stay up too late to write, so I'm tired the next day, which means it's hard to get the energy to write, which means I stay up too late in order to get the writing in. That's just not good.