The brain, like muscles, consumes glucose and ATP (the body's energy molecule) to function.
And at some point, from the strenuous mental process, ATP runs out. This doesn't affect the muscles and other organs because they have their own energy depot.
However, the brain notices the lack of energy. This is manifested by distraction, inability to concentrate, temporary deterioration of memory.
This condition is stopped by a hearty lunch and a healthy sleep, after which the brain "reloads" and is ready to function again in its usual format.