You know you're tired when you're driving and you blink and a road sign appears, but it felt like you just blinked. (While going through Kansas on a cross-country run once I blinked and the sun came up, I must have been out at least an hour, Kansas is the only place you can do this and not die.)
Basically any time you think an idea is so brilliant you have to write it down right now, you're really tired. (I've recently "invented" the portable typewriter, centrifugal turbines, carmax, and the boondock saints this way, only to realize that they already exist after reading my scribbles in the morning.)
You're really tired when you get paralyzed by confusion contemplating everyday items, words, and concepts.
You're in the final stages when have full-on multisensory hallucinations. Hopefully the figments of your imagination are good enough friends to take your car keys from you, like mine are.
I'm pretty sure that in lab sleep-deprivation experiments the stage following hallucination was death, so you really can't get much more tired than that.
"To the intransigent mind, to the inviolate truth"
I don't really have "mornings", so much as "times when I switch from booze to coffee"