Adenosine causes humans 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 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)
During this breakdown Adenosine is produced continuously and starts accumulating. They accumulate in the Adenosine receptors such as A1, A2a, A2b and A3 (as shown in fig below).
But not all the receptors induce sleep when they receive Adenosine and inhibit signal propagation. Mainly "A1 receptor" when receives adenosine induces sleep i.e. A1 receptors are implicated in sleep promotion by inhibiting wake-promoting cholinergic neurons in the basal forebrain.
This happens continuously and inhibition of A1 receptor by Adenosine slowly induces sleep by relaxing body muscle but not causing weakness!
Then how this accumulated Adenosine is metabolized ?
When we fall asleep at night 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)