You are never tired! You just lost your motivation...

 
  When you are motivated and when you are in the Flow state, you do not feel tiredness at all. Sleep is just an interrupt that is not letting you do what you were doing.
Simply put motivation is like a force. ( let the force be with you ..;)
Most of the time, a lack of motivation is experienced, especially in the work-life, due to a missing or incomplete enabler:

• Mastery  ( you should master what you are doing, the more mastery the better )
• Autonomy ( self governance. you chose what to do )
• Purpose ( why are you doing what you do, what's the meaning of it,does it matter? ) 

 More on this in the following TED Talk: ( watch TED and not TV!)

Your Purpose depends on your needs.( the unfulfilled ones!)  For sure you know Maslow's pyramid. You go from bottom to top, you kind of have to fulfill the current level before you go to the next. ( sounds like a computer game, right..? the game designers have to keep you motivated to play the game.. ).

I feel tired now.. it is 11:35 PM, time to sleep, the brain needs to do its housekeeping.