Too long sleep withdrawal can cause death.
Where I once worked nightshifts there was a story of one not sleeping in four
days (might not have been that sober, either), and then dying.
I forget how old he was, maybe in his thirties?
But I did not know him; I just heard the story.
Also some rumours concerning the US army in the past forcing too much sleep
withdrawal on persons.
>how can we miss such an important function of sleep if we want to understand
the brain?<
I am not your "we".
I do not recall a real serious interest to imderstand all about about a brain,
nor believe that that is possible for a human now.
also I have my own theories on various sleep aspects, but that does not mean
that I have to feel excessively like sharing them here.