It follows your real nights, not the clock you wish you kept. The plan is anchored to the time you actually woke up, so a late night pushes your peaks, your caffeine cutoff and tonight's bedtime later too.

Sleep debt is how far short of your sleep need you’ve fallen across the last 14 nights. Nights you sleep past your need pay it down.

Body clock. Core body temperature bottoms out two to three hours before you habitually wake, and melatonin starts rising roughly seven hours before that. Both are estimated from your recent nights, weighted towards the last few.

Getting back on target happens gradually. Bedtime moves at most an hour per night, because that is about as fast as a body clock will follow.

Energy uses the two-process model: a 24-hour circadian drive minus sleep pressure that builds while you’re awake. Sleep debt raises your starting pressure, and sleep inertia dulls the first hour after waking.