Why jet lag can feel worse when you travel from west to east

Most of our internal clocks are a bit slow and want to have a longer day

Jet lag may be the worst part of travelling. And it hits many people harder travelling east than west. Why they feel this way is unclear. But scientists recently developed a model that mimics special time-keeping cells in the body and offers a mathematical explanation for why travelling from west to east feels so much worse. It also offers insights on recovering from jet lag.

Deep inside the brain, in a region called the hypothalamus (right above where our optic nerves cross) the internal clock is ticking. And approximately every 24 hours, 20,000 special pacemaker cells that inhabit this area, known as the suprachiasmatic nucleus, synchronise, signalling to the rest of the body whether it's night or day. These cells know which signal to send because they receive light input from our environments - bright says wake, dark says sleep.

But when you travel across multiple time zones, like flying from New York to Moscow, those little pacemaker cells that thought they knew the routine scramble around confused before they can put on their show. The whole body feels groggy because it's looking for the time and can't find it. The result: jet lag.

Most of our internal clocks are a little bit slow, and in the absence of consistent light cues - like when you travel across time zones - the pacemaker cells in your body want to have a longer day, said Michelle Girvan, a physicist at the University of Maryland who worked on the model published in the journal Chaos.

"This is all because the body's internal clock has a natural period of slightly longer than 24 hours, which means that it has an easier time travelling west and lengthening the day than travelling east and shortening the day," Girvan said.

Jet lag can be resolved by matching your internal clock to your destination's clock as soon as possible. (There's an app for that). So the researchers built a model that considers all of your pacemaker cells, how sensitive you are to light, the brightness of light, multiple time zones and people's slightly off kilter internal clocks. They hope the model offers a simple way of explaining how a typical body might recover from jet lag with no intervention. That is, how its pacemaker cells try to synchronise in the presence of different light cues (like sunshine, artificial light or dim light from clouds) when arriving at various time zones three hours, six hours, nine hours and 12 hours away either to the east or west. The model confirms what was already known: generally, westward recovery is easier than eastward. But it also helps us understand that flying across more time zones can sometimes be easier than travelling across fewer.

For example, it would take you about eight days to recover from a westward trip across nine time zones, if you did nothing to fight it. But if you cross the same number of time zones going east, recovery would take more than 13 days, according to the model. This recovery time is worse than if you flew smack across the globe, crossing 12 time zones, which is about the distance from New York to Japan.

Confusing? The model shows that your body is confused, too, as your cells try to adjust to new light cues in different places. It also shows that a trip less than 12 hours going east is going to feel worse than the same time going west.

It all comes back to whether you're a lark - an early riser - or an owl, and "most people are a little owlish," said David Welsh, a neuroscientist at the University of California, San Diego, who studies the body's pacemaker cells and was not involved in the study.

If you're travelling across several time zones, like from New York to Moscow, and you want to start feeling normal sooner, "you really want to experience that external stimuli appropriate to your new time zone as quickly as possible," said Girvan. And that sometimes means owls succumbing to early nights.

© 2016 The New York Times