Explainer: What is jet lag?

Researchers don’t fully understand how the body’s internal clocks coordinate. But they do know your body will notice when its circadian rhythms are mismatched to the time zone they’re in.

“There are a lot of different hormones and other systems in the body that under normal situations are internally synchronized,” says Shawn Youngstedt. He studies circadian rhythms at the University of South Carolina in Columbia. The body clock times when those chemicals are released to orchestrate biological activities. Crossing a lot of time zones can jumble these systems. Hormones and the activities they direct may now operate at inappropriate times.

People will recognize this because they develop what is called jet lag. Its symptoms include being tired — and sometimes having an upset stomach.

Consider a runner who has just flown from Denver to Paris. The athlete lands in a city that’s 8 hours ahead. In attempting to adjust to this time difference, the runner’s hormones must change their routine. For example, the rhythm of cortisol — a hormone that is influenced by sleep and stress — might be produced earlier than normal. But the body may delay the release of another hormone. So the rhythms of these two hormones suddenly are misaligned — and the poor traveler feels groggy.

Hormones are just one class of the chemical reactions that maintain life in every cell. Collectively, these reactions are known as the body’s metabolism. Those reactions vary a lot over the course of 24 hours, but their schedule on any given day is little different than on any other. And any or all may become perturbed by travel across many time zones.

An athlete may eat when his body knows it’s supposed to be sleeping, for instance. Our metabolism is wired to receive food at mid-day. But say a flier eats a burger on a plane at midnight. “You’re forcing your metabolic cycles to work at the wrong time,” notes Paolo Sassone-Corsi of the University of California, Irvine. So the hormones needed to signal digestion end up confused. And this may upset the tummy.

Power Words

biological clock A mechanism present in all life forms that controls when various functions such as metabolic signals, sleep cycles or photosynthesis should occur.

circadian rhythm Biological functions such as body temperature and sleeping/waking times that operate on a roughly 24-hour cycle.

hormone A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body. 

jet lag A temporary disruption of bodily rhythms caused when someone travels across several time zones in a matter of hours.

metabolism The set of chemical reactions that maintain life in living cells.

synchronize To work together in harmony at the same time or rate, like in a marching band.