usually when I don't get enough sleep, I feel sleepy, all day.

for some reason today was different.

slept for less than 4 hours

no coffee, etc

feeling alert AF.

Sleep cycles or something.
When I sleep around 5.5 hours, I feel super woke when I wake up. But the effect lasts 2 or 3 days, I need an average of 6.5 hours of sleep or more to not be a zombie.
I really want to study my sleep patterns to maximize my uptime, I've reduced my average hours of sleep from 8-9+ hours when I was around 15 to... * checks Mi Band * exactly 6.5 this week. It's been a gradual process but I am very happy with what I achieved and the limits I've found. Makes me feel more in control of my time.
Last night I slept 5.5 hours and I woke up pretty easily, more than last few days, so my hypothesis checks out with this data.

your brain thinks "ok wtf is going on, 4hrs of sleep and NO COFFEE, something really bad must have happened"

Been there man, it's adrenaline. It is going to wear off tomorrow and you are going to feel like crap.

Wait for the crash. It'll come.

@c3r38r170 do not sacrifice sleep for bullshit “sleep less do more” life advice. Seriously, your eyes willtell everyone that you are destroying your life.