An object at rest remains at rest, and an object in motion remains in motion

If you want something done ask a busy person to do it.

I think it is generally like this for most people: Once you get going (do something), it is much easier to then also do something else.

My guess would be on the days you work, your body can stay in a working mode/mindset, whereas when you try to do so on non-work days, you have to force yourself into that mindset, which requires a lot of self control. It also might be your body telling you to take a break, and you’re overriding that feeling by writing.

Because your brain only uses words for a small fraction of its processing, but the things being processed span across both parts of your brain. As you work with your body, your brain is doing math and gaining insight about the world. It’s helping to process information involved in the things you write about.

Non-verbal activities are an excellent way to keep working on problems you originally conceived verbally, when it’s time for that part of you to take a break.

And just basic interaction with the concrete world is in some ways like consulting a computerized oracle that, while speaking in a strange tongue, never says anything untrue.

Also exercise gets your blood moving. Best way to get your brain well-supplied with oxygen, nutrients, and growth hormones is to exercise. It’s as good as coffee.

Because you probably would rather write than be doing your job?

What does your writing mean to you? Can you find a meaningful relationship between what your writing means to you and what your work means to you?

Similarly, what do you weekends mean to you? Can you find a meaningful relationship between what your writing means to you and weekends?

I have a couple of ideas. One is that variety promotes creativity. The other is that you might be thinking more deeply about your writing while you are away, but while you are forcing yourself to do it you are pushing it away.