Two clocks decide when you feel sleepy. The first is sleep pressure: a molecule called adenosine piles up every hour you're awake and drains while you sleep. The warm solid curve is that pressure.
The second is your circadian clock — a fixed 24-hour wave (the gold dashed line) that you can't reschedule. It bottoms out before dawn and peaks in the evening, no matter when you went to bed.
You fall asleep easily only when pressure is high and the circadian wave is low — the two lining up. Misalign them and you get grogginess, or an evening "second wind" where the rising wave hides a debt that's still there.
Puberty shifts a teen's whole wave about two hours later. Flip the chronotype and watch the gold wave — and the dashed "ideal" window — slide right. That's why an early school bell fights a teenager's biology.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.