Sitting still for a long stretch slows circulation, which reduces oxygen flow to the brain. The effect isn't immediate, but as hours pass, attention quality noticeably drops.
So while locking into the same task for an hour straight feels intuitively productive, coming back after a short standing break usually produces better results.
Standing up every fifty minutes for a few steps, even switching the sound in the room while you're at it, refreshes both body and mind. It's a small habit, but it's what actually makes the difference on long days.