That's the most normal thing in the world.

We aren't machines - most people can only focus deeply for at most a couple of hours at a time, and it takes a lot of energy to focuslike that. Just like you can't run a whole marathon at sprinting-after-the-bus speeds, you won't be able to work a whole day at the level of your peak performing hour. Best thing to do is plan your work accordingly.

The number one thing for me is taking breaks BEFORE I think I need it. Way more frequent breaks than you think.

Short breaks so you don't lose your flow, but still take time to get some blood flow through your body up to your brain: Set an alarm every half hour or so for a mini break 30-60 sec (closing your eyes, stretching, breathing, standing and staring out a window). Every hour move away from the desk 2-5 min (refill your water and grab a fruit, or empty the dishwasher or quick walk up and down the stairs or around the building for some fresh air). Halfway between morning and lunch, and halfway between lunch and night take a longer break 15 or so minutes (with a cup of tea or coffee and reading or chatting with colleagues or a friend).

Absolutely no (constructive) work talk during breaks with colleagues. Better to sit in silence than ruin your break time.

Brisk walk (get the heart rate up) or quick nap during lunch.

Save some menial tasks for when you run out of steam, usually after lunch and often towards end of day. Replying to a few emails or checking off some small tasks - something simple just to get the ball rolling after lunch or to wind down the work day.

Make sure you eat and drink properly.

Avoid negative self-talk, because you're doing really well with your human brain and biological need for rest and recuperating.

And ask for help even when you don't need it, because sometimes just explaining your work means you find a solution that would have taken a lot longer to find in silence, and sometimes it just makes someone else feel good to be of assistance.