I actually did research on the topic for learning in another area that I was working in. Studies have shown that the brain works at optimal performance for twenty minutes.then starts to slow down or wander. Push too hard and it will rebel. ( my words not the scientific jargon.
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You are doing the right thing. The studies agreed that simply take a five minute break every twenty minutes to get optimal performance results.
Just change your focus, drink some tea, design a different pattern or maybe read an article.
You are doing fine it's not a problem it's very normal and it demonstrates that actually you are functioning well.
So go ahead and enjoy your long sessions just add in even more. LOL.