My brain is full.
It's okay, as soon as I get a chance, I'll get some filing done and these piles of ideas and concepts will be stored away in some tidy place of my mind, and there will be room for more.
But it's times like these, when I'm working through some really complicated programming at the same time I'm working through some really twisted inter-company politics that I have to use all my brain at once. Not just rote memorization, but a fundamental understanding of the logic and reason that goes into a project as well as a fundamental understanding of the really screwed up emotional reasons people are doing the crazy things they're doing.
It's literally a physical sensation that starts in the back of my head and works its way around to my frontal lobes. Maybe it's blood flow, maybe it's psychosomatic, but there's a buzzing in my head that feels almost like a swarm of bees.
Like any workout, water is important. And rest. You can't really stretch your brain (although I obviously stretch my imagination), but you can "walk it off" and get some blood flowing into the nooks and crannies of the brain.
And, like any hard, physical training, the second day is harder. The poor organ is tired (I tell ya, getting up this morning was no picnic). Fortunately I'm no stranger to endurance sports, but at some point you have to know your limits and be able to stop, sit down for a minute and admit, "My brain is full."
And then, hit it all over again until you fill up your brain again.