A flight from Paris to New York is easier on the brain than one from New York to Paris, according to a new study that finds jet lag is based not only on distance traveled, but also the direction of travel.

In the journal Chaos, researchers from the University of Maryland present a mathematical model that considers the brain's "neuronal oscillator cells," which regulate a person's biological clock, or circadian rhythm.

Though everyone's clock is different, it tends to operate on a schedule slightly longer than 24 hours, study author Michelle Girvan explains, per Live Science.

That means the brain responds better to a longer day achieved by flying west across time zones than a shorter one achieved by flying east. Researchers say a person flying west across three time zones can recover in less than four days, while a person flying east would need a little more than four days.

The divide becomes more apparent across more time zones. A person flying west across nine time zones would need a little less than eight days to recover, compared to more than 12 days for those flying east—though the recovery for crossing 12 time zones is about nine days either way.

The mathematical model—which considers external cues like sunlight and geographical latitude, per CNN—was based on a circadian rhythm of 24.5 hours, and some people "may have longer or shorter natural rhythms," Girvan says.

But overall, the study shows that the general rule of giving yourself one day of recovery for every time zone crossed is off base. Girvan says researchers hope the model will "serve as a guide for developing more in-depth qualitative approaches, as well as strategies to combat circadian rhythm disruptions due to rapid cross-time-zone travel, shift work, or blindness," per Science Alert.

(This trick might stop jet lag before it starts.)

This article originally appeared on Newser: Flying East Is a Pain for Your Brain