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Our Biological Clocks

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The expression “jet lag ” is sometimes mistakenly used to refer to any discomfort experienced during a long airplane trip. Certainly, after a 10-hour north-south flight from Germany to South Africa, for example, passengers may feel very tired because of sitting still for so long, breathing the dry air on the plane, and so on. But the true symptoms of jet lag occur only when people make flights across multiple meridians of longitude. Someone who makes a 5-hour flight from New York to Los Angeles will therefore experience much more trouble over the next few days than someone who has made that 10-hour flight from Germany to South Africa, which are only one time zone apart.


The alternation of day and night is the result of the Earth’s rotation on its axis. One complete rotation of the Earth takes 24 hours, the length of one day.

This constant rotation of the Earth upon its axis posed a problem when efforts began to create a single time-reference system for all people everywhere on Earth. If noon were defined as the time when the sun reached its highest point in the sky, then noon would occur at a given time for people at any given location, but a few seconds later for people at another location just a few metres farther west, and several minutes earlier for people living a few dozen or a few hundred kilometres farther east. The only people who lived at different locations but experienced noon at the same time would be those who lived north or south of each other along the exact same meridian. All of this would obviously be pretty impractical if people were trying to created a unitary time-reference system.

To solve this problem, time zones were created. Time zones are a convention according to which all locations between two meridians separated by 15 degrees of longitude use the same time. Why 15 degrees? Simply because the circumference of the Earth comprises a total of 360 degrees of longitude, and 360 degrees divided by the 24 hours in a day equals 15. The standard time for any given time zone equals the local solar time at the central meridian in that zone (the dotted vertical lines in the diagram below).

This system was first proposed in 1876, with the meridian that passes through the city of Greenwich, England as the reference for the other time zones. The imaginary line where the date changes (the International Date Line) thus runs through the Pacific Ocean, for the most part following the 180th meridian of longitude.

Time zones make it easier to state the arrival time, in local time, for trips over long distances in today’s rapid jet airliners. For instance, suppose an airplane takes off from Paris, France at 10:00 AM local time and takes 7 hours to make the flight to New York. In the process, however, it “flies back”through 6 time zones, so that its passengers arrive in New York at 11:00 AM local time, only 1 hour later than they left Paris.


If they don’t go to bed that night until late evening New York time, these passengers will have been awake for 6 hours longer than their hosts and so feel very sleepy long before bedtime. For the same reason, many of these travellers will wake up in the middle of the night, ready to start their day: their bodies’ biological clocks will still be operating on Paris time. These people are experiencing the disagreeable phenomenon known as jet lag.

The reason that we experience jet lag is that our bodies have evolved for hundreds of thousands of years according to the natural rhythm of the 24-hour day and have adapted to it in many ways. In fact, this is the entire subject of the field of chronobiology.

Evolution never foresaw that in the course of scarcely a century, people would invent airplanes and start making long-haul flights routinely. In nature, the length of the day changes with the season, but does so imperceptibly, over several months. When people fly from one continent to another, their day can be shortened or lengthened by several hours all at once!

The result is the many symptoms of jet lag, which reflect the work that the body has to do to resynchronize its various biological clocks in response to this sudden time change.

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