Adenosine is a waste product of neurons and it is a potent nervous system depressant which is why if we don’t sleep for a long time we hallucinate, start to feel cold, and feel tired. We feel like we’re drugged because we kind of are. It also increases sleep pressure. Unlike other areas of the body the brain has no way of flushing out the adenosine unless we sleep which activates the glymphatic system. if we don’t sleep the adenosine keeps accumulating and it can kill us in a similar way an od of sleeping pills can.
I couldn’t do it as an adult, but two separate times as a kid / teenager I stayed up for 6-7 days straight.
The first time I didn’t have any negative symptoms except nausea and, obviously, fatigue.
The second time I did have mild visuals starting around the fifth day. It was decades ago, so I only really remember one, which was goblin or green witch-like clawed fingers curling around my doorframe. I think I blinked hard a few times and did the stereotypical cartoon head shake and they were gone.