You would like to keep working on something until it's done so you keep drinking coffee and keep working. But eventually you get to a point where your eyes are open but you are just sitting there, staring. Brain not working.
Bonk.
You need a ZenSlap.
Also, which side of the pond are you on? "Bonk" has two meanings over here... http://www.dict.org/bin/Dict?Form=Dict2&Database=*&Query=bonk
It's mountain biker slang for what happens when you attempt a heroic feat of exertion and run afoul of your own physical limitations in a dramatic way. Illustrated in a recent advertisement in which the biker is climbing a steep hill then appears to hit an invisible wall and begins rolling backwards downhill. "Bonk" is the sound your head makes when it hits the wall.