Jet lag sucks. Finally get to the other side of the world for that adventure of a lifetime? Great, now prepare to suffer through several days of your confused body yelling, “What the hell are you trying to do to me?!” For some reason, that lagging sensation is much worse when flying east, and now science has told us why.

The journal Chaos has just published a study explaining this directional phenomenon. Our circadian rhythms function according to light exposure, which, theoretically, will stay about the same if you stay in the same time zone. The average circadian rhythm, though, clocks in at a little over a day (24.5 hours) — our bodies are happier when chasing the sun. The sun sets in the west, so our days are slightly elongated when flying in that direction.

Scientists used mathematical models to simulate our circadian adjustments when moving through time zones. “Our model explores what would happen to an individual it he/she were suddenly taken from one time zone and dropped in another,” Michelle Girvan, an associate physics professor of physics at the University of Maryland told Gizmodo. “The important 30 minute difference that comes into play is that the natural frequency of [the brain cells] is about 30 minutes longer than 24 hours.”

When following the sun, they found that the effects of jet lag would usually last about a day per zone—i.e. travel three time zones, spend three days in lethargy. Flying the other direction increases jet lag time by about 33%—three times zones will take four days, six time zones will take eight days, etc.

The experiment also suggests that our circadian rhythms are all different, so we’ll all experience jet lag in our own way. “Our model suggests that the difference between a person’s natural period and 24 hours controls how they experience jet lag,” Girvan said in a statement.

So, jet lag still sucks, but at least now you know why. When in doubt, follow the sun.

Image: Jason O’Halloran