What causes adenosine build up in the brain when awake?
Adenosine causes humans to become sleepy. But how ? 


During day time we consume food which is broken down into glucose. This glucose is broken down by "Glycolysis" in cell's cytoplasm during which ATP is produced. This produced ATP is is then used by body as an energy supplier. ATP breaks down into ADP and then AMP with the release of energy which our body consumes for doing work. 




(Fig 1- Structure of Adenosine to ATP)

Adenosine is produced continuously and starts to accumulate around and in Adenosine receptors such as A1, A2a, A2b and A3 (as shown in fig below).Adenosine inhibit signal propagation and energy production in at least A1. (Not all the receptors induce sleep when they receive Adenosine.) "A1 receptor" receives adenosine and induces sleep. A1 receptors are implicated in sleep promotion by inhibiting wake-promoting cholinergic neurons in the basal forebrain.



Continuous inhibition of A1 receptor by Adenosine slowly induces sleep by relaxing the brain and muscles but without substantially weakening their abilities!

How is Adenosine metabolized ?

During sleep the accumulated Adenosine is metabolized by Adenosine deaminase enzyme which catalyzes the irreversible deamination of 2'-deoxyadenosine and adenosine to deoxyinosine and inosine. With the reduction in the Adenosine content the body is excited from sleep slowly.

So in short summarize:


  The accumulation of adenosine during waking periods is thus associated
  with the depletion of the ATP reserves stored as glycogen in the
  brain. The increased adenosine levels trigger non-REM sleep, during
  which the brain is less active, thus placing it in a recovery phase
  that is absolutely essential—among other things, to let it rebuild its
  stores of glycogen. 
  
  Because adenosine is continuously metabolized by the enzyme adenosine
  desaminase, the decline in adenosine production during sleep quickly
  causes a general decline in adenosine concentrations in the brain,
  eventually producing conditions more favourable to awakening.


(Refer this page: MOLECULES THAT BUILD UP AND MAKE YOU SLEEP)