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.
Fatal insomnia is a misnomer. It’s a prion disease that literally eats away at the brain including the part of the brain that controls respiration and temperature. My point is the adenosine waste levels in the brain (fun fact, adenosine is what makes us feel sleepy or even start hallucinating if we’ve been awake too long, caffeine blocks the receptors of this waste product) would cause unconsciousness before they became fatal. I’ve never heard of anyone not being able to go unconscious.