Jet Lag Calculator

The rule of thumb most sleep researchers cite is one day of recovery per time zone crossed. The tool flags a trip of 4 hours difference or less as mild, 5-8 hours as moderate, and 9+ hours as severe. A flight from London to New York (5 hours behind) typically takes 3 days to fully shake off, while London to Tokyo (9 hours ahead) can take 5-7 days. London to Sydney is the worst-case for UK travellers, regularly taking a full week to settle.

Direction matters more than most people realise. Eastbound travel is harder because you are asking your body to fall asleep earlier than its internal clock thinks it should. Westbound is gentler because staying up later feels more natural; the human circadian clock runs slightly longer than 24 hours, so it adapts more easily to a longer day than a shorter one.

Light exposure is by far the biggest lever. Get outside in natural daylight at the right time for the local clock and your circadian rhythm resets faster than any supplement will achieve. Eastbound travellers want morning light on arrival day; westbound travellers want afternoon and evening light. Skipping this and going straight to a hotel room with the curtains drawn is the single most common reason jet lag drags on for an extra few days.

Caffeine and naps both work but timing matters. A 20-minute nap on arrival day is fine; anything over an hour wrecks the next night's sleep. Caffeine has a 5-6 hour half-life, so a 15:00 coffee is still partly active at 21:00. Save your last coffee for around lunch on adjustment days. For longer routes, plan around the [Timezone Meeting Planner](/timezone-meeting-planner) before scheduling any work commitments.