It’s believed we need sleep because of a waste product called adenosine builds up in our brains while we’re awake. When we’re unconscious the waste stops building up and the CSF in the brain gets rid of it. Adenosine is a CNS depressant and works on the same receptors as sleeping pills. Eventually your nervous system would be so depressed you’d just fall unconscious as if you took some sleeping pills.
From what I’ve heard it is actually hard to fall asleep after an extended time of being awake. I doubt the one mechanism discussed here is enough to explain all that is going on while sleep deprived.