I woke up in the middle of the night last night/this morning and could not freaking fall back asleep. My mind kept drifting and fixating on recall-memory sorts of things, like the classification of vegetables. It reminded me of when I was taking a high school anatomy & physiology course, and I used to lie awake counting the bones in my body or thinking about the different components of blood. Good for when it was time to take tests, but bad for sleep.