Sleep wall • noun • that feeling you get when you feel like you're going to fall asleep but then it passes and you're fine. Often afflicts those with PopTart deficiencies. Those who do not suffer from sleep walls are known to be excellent spellers. Causes include watching too much Dexter and Breaking Bad. Tennis players are at high risk.
"Dude, you hit a sleep wall. You should eat some PopTarts and stop watching so much Dexter..."
"I do not have sleep walls, because I am good at anagrams and can define the word 'retrogress'."