Hours between your body clock and the local clock, day by day. Zero is fully adjusted — and the gap between the two lines is what the light timing is actually worth.
Jet lag isn't tiredness — it's a timing problem. Your body runs an internal clock that was set at home, and it doesn't care that the aircraft moved. This builds the light, dark and sleep schedule that drags that clock to local time as fast as it will go.
Each bar is one night in bed. They march across the clock a step at a time — which is the whole point, because a clock moved gently at home is a clock you don't have to drag on arrival.
Why this is the highest-value hour you'll spend: every hour shifted at home is an hour you don't spend shifting abroad, where the light is working against you and you have somewhere to be. Two or three days at an hour a day is usually all anyone can sustain around work and family — and for most trips that's already a third of the job done. Pair each step with light: on the mornings you're advancing, get outside as soon as you're up; on the evenings you're delaying, keep the lights on late.
The cheapest jet lag treatment is the one you take before leaving. Shift your bedtime and wake time toward the destination for a few nights and you step off the plane part-way adjusted.
What an hour of bright light does to your clock, depending on when you get it. Above the line it pulls you earlier, below it pushes you later, and the crossover sits exactly at your temperature low — which is why the same walk outside helps at 7am and hurts at 4am.
The rule underneath everything: light landing in the hours before your temperature low pushes your clock later; light after it pulls your clock earlier. That single fact explains why night owls who scroll in bed get later and later, why an early flight east feels worse than the same distance west, and why the fix for both is a walk outside at the right hour rather than more coffee.
Light is the strongest input your body clock has — stronger than melatonin, meals, exercise or willpower. Timing decides whether it helps or hurts.
Debt climbs a fixed amount every short night and comes down only as fast as you can add sleep — and since you can borrow far quicker than you can repay, the climb is always steeper than the descent.
Sleep debt is real but it is not a bank balance. Short nights accumulate a measurable deficit; recovery sleep is deeper and more efficient than ordinary sleep, so you repay with less than you lost — just not in one weekend.
Caffeine leaves on a half-life, not a schedule, so an afternoon cup is still measurably present at midnight. The shaded block is when you're trying to sleep through it.
Two levers decide most of tonight: when you stop drinking caffeine, and whether you go to bed at a time that lets you wake up without an alarm dragging you out of deep sleep.