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Rule of thumb: about a day per zone travelling east, roughly two-thirds of that going west, because the body clock naturally runs slightly long and delays (staying up later) more easily than it advances. Under 3 zones barely registers for most people.
What The Jet Lag Calculator Works Out
- Typical recovery: roughly a day per time zone travelling east and two-thirds of that travelling west. The body's timekeeper, the suprachiasmatic nucleus, resets only an hour or so a day, and because the human clock naturally runs slightly longer than 24 hours, delaying (westward) goes with the grain while advancing (eastward) fights it. The UK-to-Japan trip wrecks a week that the same distance to California merely dents.
- Light strategy: light is the reset lever everything else assists. Eastward travellers chase morning light and shun the late evening's; westward travellers do the reverse. Dawn light hitting a body still on home time's small hours pushes the clock the wrong way, the classic error.
- Arrival date and fully adjusted by (advanced): enter your arrival date to get the calendar day by which you should feel normal. Under about three zones the effect fades into ordinary travel tiredness, and north-south flights produce fatigue but no lag at all.
- Bedtime shifting before you fly (advanced): each day of moving bedtime an hour towards destination time before departure removes an hour of lag on arrival. Two or three days is realistic for most people; set watches to destination time at boarding.
- How fast you adjust (advanced): older adults, poor sleepers and small children often take longer; the slower setting stretches the recovery by about a third.
- Trip strategy (advanced): trips of a couple of days are usually best flown on home time, meetings permitting, because adjusting twice costs more than the lag. If recovery would take longer than the trip, shift only for the hours you need to be sharp. Athletes and orchestras plan arrival days per zone crossed for exactly the performance decrements the studies measure.
- Jet lag coming home (advanced): the return crosses the same zones in the opposite direction, so a kind outbound (west) means a harder return (east). The card gives the date to keep clear of anything important.
- Day-by-day plan (advanced): a target local bedtime for each day from arrival, moving an hour a day eastward or about 90 minutes westward from your home bedtime, with the light and routine advice for that day. Caffeine belongs in destination mornings only; melatonin, prescription-only in the UK, works as a timing signal at destination bedtime rather than a sedative. Cabin dehydration and the nightcap are removable aggravators, not the lag itself.
Further reading: NHS: jet lag.