I think it's low liver glycogen. Not eating enough carbs recently, and/or not replacing the carbs your liver used during the night. Your brain is a huge carb consumer. When you sleep, it's still working and gets its carbs from liver glycogen. The next day during the time before you ride, this glycogen needs to be replaced or you get the brain fog. When I have a big ride, I'll wake up 3 hours before the ride, eat a bunch of carbs (300-400 cal.) and go back to sleep. This is the reason. It's not about muscle glycogen, which won't change while you sleep.