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.''