Jet-Lag Calculator

Frequently Asked Questions

• What does the calculator work out?+
  You enter the clock difference in hours and whether the destination clock is ahead or behind. The tool reduces that to a 24-hour day and shows how many days the body clock takes to close the gap by advancing and by delaying, at the published flight averages of about 1 hour a day advancing and 1.5 hours a day delaying. The headline is the shorter of the two.
• Why is travelling east harder than west?+
  Flying east needs the body clock to advance and flying west needs it to delay. Eastman and Burgess (2009) explain the asymmetry “at least in part” by the clock’s average period being slightly longer than 24 hours: Duffy et al. (2011) measured an average of 24.15 hours in 157 people, with some individuals below 24.
• Where do the day counts come from?+
  From early estimates on real flights, reviewed by Aschoff’s group in 1975: the clock delayed about 92 minutes a day after westward flights and advanced about 57 minutes a day after eastward ones. Eastman and Burgess (2009) work these through as 1.5 hours a day and 1 hour a day, and say they are “altered by the actual pattern of light and dark”. Every day count here is at those published averages, not a prediction for any one person.
• Why does it show two directions?+
  Because a clock difference can be closed either way round. A destination 8 hours ahead can be reached by advancing 8 hours or by delaying 16. At the published averages the two take the same time at a 9.6-hour advance, so for larger advances delaying is the shorter way. That is also why 12 hours east and 12 hours west give the same answer: they are the same clock change.
• What happens on a very large eastward shift?+
  The body does not always advance. Eastman and Burgess (2009) write that antidromic re-entrainment, adjusting by delaying, “is common following eastward travel, especially when 8 or more time zones are crossed”. Gundel and Wegmann (1989) estimated the critical advance at +7 to +10 hours, and seven of eight men in a Tokyo team’s study delayed after an 11-hour eastward flight to New York (Takahashi et al. 2001). The tool flags advances of 8 to 12 hours for this reason; the samples are small, so it does not predict which way a given body goes.
• Why hours of clock difference instead of time zones crossed?+
  The body adjusts to the clock it finds on arrival, so the unit is the difference in hours. Some zones are half an hour off (Singapore to Delhi is 2.5 hours), and daylight saving at the destination moves many routes by an hour: Singapore to London is 8 hours in January and 7 in July. Check the difference for your travel dates, because Singapore, Malaysia and Hong Kong do not change their clocks while many destinations do.
• What does light do to the body clock?+
  In a study of 21 people, Khalsa et al. (2003) found that bright light centred before the body’s temperature low point delayed the clock and light centred after it advanced it. The tool shows where a textbook low point of 04:00 home time falls on the destination clock. Individual low points vary, so this describes the mechanism rather than giving a schedule.
• Is the day count when I will feel normal?+
  No. It is the average time for the body clock to shift. In 64,847 trips by Oura Ring users starting in North America and Europe, Willoughby et al. (2025) found sleep duration back within about 12 minutes of baseline after about 2 days, while sleep timing had often not returned to baseline by day 15, the end of the study window. That measures sleep timing, not the body clock directly, and the trips did not start in Asia.
• Is this medical advice?+
  No. It applies published averages to a clock difference and describes what studies found. It gives no advice on medicines, supplements or fitness to travel. Anyone with a sleep disorder or a health condition should talk to a doctor.
• Is the Jet-Lag Calculator free?+
  Yes, with no account or usage limit. It runs entirely in your browser and sends nothing you enter anywhere.