My workdays are usually wake-to-sleep every day of the week because I am stubbornly committed to catching up on my backlog.

I'd reccommend aiming for a four day work week. Break up your work days with a 90-minute break away from any screens after 3 - 5 hours; ideally with exercise or outside time.
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If you cannot get into the flow for a particular project that day, jump to another one that feels easier to work on for a bit and come back to the first one later. Don't put it off perpetually; working on it in smaller bursts is better progress than none and may help delay the onset of burnout.
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Making sure you get up and moving periodically [AWAY from a screen]. I have a hula hoop in my office that I use every couple hours to help stretch my spine and legs while also getting my blood going. Sometimes I might swap it out for a jump rope.
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