When you find yourself just staring off in space, and there is this low grade buzz in your head that has been there for a while but you just noticed it--it's time to hit the sack. When your thoughts keep circling like buzzards around a dead carcass, and you can't remember what you were trying to remember or why you needed to remember anything--it's time to call it quits. When someone says something to you and you hear the words but you can't put the meaning together because you're trying to just process the sounds of the words and meaning is beyond your brain's functioning level at the moment, it's time to stop trying to think and start trying to doze off. Some days are like that. You are too tired to know you are tired, and if you're lucky, some kind person will take you by the hand and lead you to your place of repose, tuck you in, and turn out the lights.