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.
Its not the lack of sleep that kills you, it is the stress on your body from not sleeping for long periods of time causing things like heart failure and aneurysms.