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There are many reasons to feel unwell after a flight, including the stress of preparing for a trip, and the strain of flying.

But by far the biggest adjustment for our bodies is the sudden shifting into a new time zone with new patterns of light and darkness. Light-dark patterns are used by a region of the brain (the suprachiasmatic nucleus) to keep our body clock in tune with our day-night cycle. At home, only a small daily recalibration of our clock is required (about 10 minutes). However, a transatlantic flight requires a shift of several hours and it cannot be done overnight.

If we were taking an ocean liner across the Atlantic, a journey of eight days, our internal body clock would adjust day by day in approximate synchronization with the nautical time zones. But it is far more likely to fly than sail to Europe these days, and our body cannot adjust so quickly.

It takes about one day for the body clock to shift one hour when travelling eastward. Here’s a drug-free way to adjust your body clock fully or partially to local time before you go.

(Note: The method depends on the Canadian time zone in which you live. Paris, for example, is five hours ahead of Halifax, six hours ahead of Toronto and Montreal, and nine hours ahead of Vancouver. Let’s start with calculations based on living in the Eastern Daylight Time zone. If you live in a time zone east or west of EDT, you can take these principles and extrapolate; I’ll show you how below.)

Shifting your sleep-wake rhythm to Paris time

Say you normally go to sleep at midnight and rise at 8 a.m. and you want to do the same in Paris. You need to adjust your sleep-wake rhythm so you are falling asleep at 6 p.m. EDT and rising at 2 a.m. EDT. You would start six days before you go, advancing your sleep-wake rhythm by about one hour a day.

On Day 1 you would go to bed around 11 p.m. (or later if not sleepy) and get up at 7 a.m. Just after rising, get outside into bright light or sit under bright household light for 30 minutes. (Don’t look directly at the light.) In the evening, avoid bright light before bed. Use the chart below to adjust your bedtime and rise-time each day. Each day, use bright light for 30 minutes as soon as you wake up and be in relative darkness for about two hours before bed. Remember that the pattern of light and dark is your main rhythm-shifting agent.

The downside is that for a week, your friends, co-workers and family may not see much of you. The upside is arriving at your destination ready to explore. You can also choose a happy medium of advancing your sleep-wake rhythm by three hours before flying and allowing the body clock to advance the last three hours in Paris.

Flying home

Thinking about returning home to work is not nearly as fun as thinking about flying off on vacation. However, it deserves some consideration. Most people find the adjustment to westbound destinations easier because it involves delaying the sleep-wake rhythm. Our intrinsic body clock has a period that is slightly longer than 24 hours (about 24.2) so it naturally delays when it can. It will take about half the number of days to adjust than it did on the outgoing trip. I suggest you do the adjusting after you arrive back home. If you can arrive home a couple of days before you have to work, this will allow time for resynchronization. Use light before bed rather than the morning to help with the adjustment.

Although your trip will be bookended by daily adjustments to your sleep-wake timing, sometimes it’s worth it. Bonne nuit et bon voyage.

Dr. Judith R. Davidson is a clinical psychologist and sleep researcher. She works with the Kingston Family Health Team and Queen’s University at Kingston. She is the author of Sink into Sleep: A Step-by-Step Workbook for Reversing Insomnia. You can follow her on Facebook and on Twitter at @JudithRDavidson.