Catpone Cerberus

But after certain time, even if your legs can take it, you have to sit/lay down because your brains get's less oxygen due their high place when you are standing.

Or at least that is what I have read/heard/etc. i'm not professional in these things, or really in anything.

Or you've expended all your bodies energy for the day, using your brain and standing.

That could happen too yes.