Jet lag calculator and light planner

How this jet lag calculator works

This jet lag calculator tells you when to get bright light and when to avoid it, day by day, until your body clock matches the destination. It uses the time difference on your arrival date, so daylight saving changes are included. Your sleep times stay on local time from the day you land.

Step 1: find your body temperature low

Your core body temperature bottoms out late in the night. Sleep researchers use this low point, called CBTmin, as a marker for where your body clock sits. Eastman and Burgess put it about 3 to 4.5 hours before wake time for someone sleeping 8 hours, and use 3 hours in a worked example.¹ This tool uses 3 hours before your usual wake time.

Step 2: light on the right side of that low

Light shifts the clock in opposite directions depending on when it lands. In a study of 21 people given bright light at different body-clock times, light before the temperature low delayed the clock and light after it advanced the clock.² So after an eastward flight, light just before your low (often early morning at the destination) pulls you the wrong way, and light just after it helps. Westward is the reverse.

The planner draws a 3-hour light window on the helpful side of the low and a 3-hour avoid window on the other side, then trims both to the hours you are awake. The 3-hour width is a simplification. The real response curve has no sharp edges.

Step 3: move the low a little each day

The body clock does not jump. Early studies of travelers found it shifted about 57 minutes a day after flying east and 92 minutes a day after flying west.¹ The planner rounds these to 1 and 1.5 hours. Each row moves your estimated low by that amount until it is back 3 hours before your wake time.

Why long eastward trips are planned as a delay

A 10-hour advance at 1 hour a day takes 10 days. Going the other way round the clock is a 14-hour delay, which at 1.5 hours a day takes 9 to 10 days. Clocks often do this on their own: in one study cited by Eastman and Burgess, 7 of 8 people who flew 11 time zones east re-entrained by delaying.¹ The same review cites a rule of thumb of delaying from 8 time zones east, but notes it does not suit every traveler. At the rates above an advance is still quicker up to 9 hours, so this planner switches to a delay at 10 hours east.

Pre-flight days

You can start before you leave. In a lab study, 28 young adults moved their sleep 1 hour earlier each day for 3 days. With bright light for the first 3.5 hours after waking, their clocks advanced about 2.1 hours, against 0.6 hours in dim light.³ The planner moves your sleep 1 hour a day (earlier for east, later for west) and puts the light window right after waking or right before bed. It counts only 0.7 hours of body-clock shift per pre-flight day, the rate seen in that study. The study tested eastward trips only, so the westward version is an extrapolation.

Limits of this plan

• The 3-hour rule is an average. If you are a strong night owl or early bird, your low may sit an hour or two away from the estimate.
• Real days rarely follow a plan. Flights, meetings and cloudy weather change how much light you get, and the 1 and 1.5 hour rates assume you mostly follow the windows.
• Bright light means outdoors in daylight or a light box. Ordinary indoor lighting is much dimmer. See how many lux you need.
• Talk to a doctor before using bright light therapy if you have bipolar disorder, an eye condition, or take a medicine that makes you light sensitive.

Sources

1. Eastman CI, Burgess HJ. How to travel the world without jet lag. Sleep Med Clin. 2009;4(2):241-255. PMC2829880
2. Khalsa SB, Jewett ME, Cajochen C, Czeisler CA. A phase response curve to single bright light pulses in human subjects. J Physiol. 2003;549(Pt 3):945-952. doi:10.1113/jphysiol.2003.040477
3. Burgess HJ, Crowley SJ, Gazda CJ, Fogg LF, Eastman CI. Preflight adjustment to eastward travel: 3 days of advancing sleep with and without morning bright light. J Biol Rhythms. 2003;18(4):318-328. doi:10.1177/0748730403253585