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I thought that's why most people drink coffee.

I optimally sleep ~9 hours every day and start to feel tired if I sleep less for 3-4 consecutive days. I bike to work (and generally tend to go as hard as I can, so heart rate tends to spend a lot of time in Z4 or so). I almost never drink coffee so it takes me 2-3 hours to "fully awake" but I don't think that's unnatural so I don't mind it, I let nature run its course.

I got (mostly) rid of the brain fog when I bought a better mattress. Which was rather a surprise as I thought feeling like that was simply one of the things you have to get used to when getting older.

I wonder if the brain fog is caused by sleeping more, or if something else is causing you to both sleep longer and wake up w/ brain fog on those days.