I've certainly had a few days where I sat down and realized I wasn't going to get anything done. I could feel it in my skull.
I realized that not getting more stressed about it is really helpful. Everyone has their days. Programming is hard. Just let yourself be non-productive, allow yourself to relax, grab a snack or a bite to eat.
For me, non-productive days coincide with not eating well. Do something that will make you feel good. When you feel good, it's much easier to wade back into whatever you were working on.
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