Caffeine and jet lag
Updated 2026-08-17
Caffeine’s half-life is about five hours. A coffee at 4pm still has half its caffeine working at 9pm, and a quarter at 2am. Every practical rule follows from that one fact.
When you’re shifting earlier
You’re asking yourself to fall asleep before your body is ready. Caffeine makes that measurably harder, and the afternoon cup is the easiest mistake to make because it feels harmless.
Cut off about six hours before your target bedtime — not your current one. If you’re moving bedtime to 21:00, the last coffee is at 15:00, even if you’re used to drinking one at 17:00.
Six is the evidence-backed number for a normal dose. Nine if you know you’re sensitive. The stricter eight-to-ten-hour rules you see quoted are safe but usually more than the research supports.
When you’re shifting later
Flying west, the job is staying awake into what your body thinks is the middle of the night. Here caffeine is a tool, not a hazard: a cup in your late afternoon buys you the evening you need.
The same six-hour rule still applies at the far end. Win the evening, lose the night, and you’ve traded down.
Practical notes
- Don’t quit before a trip. Withdrawal on top of jet lag is a bad bargain, and the headache costs more than the caffeine would have.
- Watch the hidden sources. Airline tea, cola with dinner, dark chocolate, some painkillers.
- Timing beats quantity. One cup at the wrong hour costs more than three at the right one.
- It masks, never fixes. Caffeine hides sleep debt; it doesn’t move your clock. Only light, timing and sleep do that.
Ray puts the cutoff on your timeline at the right hour for the bedtime you’re aiming at that day, and phrases it differently depending on which way you’re moving.
Related: melatonin timing · sleeping on the plane