[corford]

Could just be simple brain exhaustion from prolonged concentration (I occasionally feel something similar, usually after back to back heavy coding days and I'm running low on sleep).

Like others are saying, a few things that might help: stay hydrated (water, not coffee/soda etc.), get a good night's sleep ahead of intensive coding days, don't eat crap food at your desk and don't stay in the "zone" for 6+ hours without getting off your chair at least once and doing something different for 30-45 mins (ideally involving fresh air, outside of the building).