Why does jet lag happen?

NARRATOR: Long-distance fliers know the feeling well - having to battle jet lag at their destinations after having flown across several time zones. Long-distance travel like this throws off our sleep-wake cycle and our inner clock. The body has its own biological mechanism that adjusts a person's metabolism to the day's light-dark cycle. Some 15 percent of our genes are active only at specific times of the day. In the evenings and at night-time, for example, our blood has more melatonin, a sleep hormone, flowing through it, and our bodies release a surge of stress hormones to help us wake up. Normally our bodies have a steady 24-hour rhythm, or circadian rhythm. It is the same as the time it takes Earth to orbit the sun. Our bodies are not equipped to adjust this natural rhythm quickly enough when people speed around the planet. Our circadian rhythm literally just cannot keep pace.

PROFESSOR HORST-WERNER KORF: "We basically take our internal clock with us from wherever it is we set out from. Let's assume we departed from Frankfurt am Main, Germany and flew to Los Angeles. Our inner clock would still be on Frankfurt time and would be telling us it is already evening, even though it is actually well before noon. We basically have to somehow digest and cope with a time change of nine hours."

NARRATOR: The biggest determinant of our body's daily rhythm is sunlight. In the morning it triggers our internal clock to wake us up and get us going. When it gets dark in the evening our body's cells shift into night mode. The human organism has adapted to this rhythm in our millions of years on Earth. When it is disturbed, we get jet lag. Flying east, i.e. towards the sun, is often harder on travellers than flying west, i.e. in the same direction as the sun.

ANDREAS SEHR: "That, of course, has to do with the fact that the departure times are different, say, for a flight to Japan than they are for a flight to the USA and you arrive at your destination at a different time of day. It is much easier to tough it out for two or three hours in the afternoon in the U.S. before going to sleep than it is to fly east and arrive at your destination in the morning. The time gap you have to bridge is a lot longer."

NARRATOR: When travellers arrive at their destination they should try to get as much sun as possible and be active as soon as they get there. All you globetrotters out there, wait to go to sleep until the sun goes down. As a rule, the body can cope with an hour's time change a day, although the time it takes people to adjust can vary a lot from person to person.