Jet lag is no fun, and a new study shows it’s even less fun when traveling from west to east, The New York Times reports. Scientists seeking to provide a mathematical explanation for the phenomenon, long noticed by travelers, built a model that mimics the body’s timekeeping cells, using factors like light sensitivity, the brightness of the light, and how much people’s internal clocks are “skewed” from real time—an offset that can be as long as 1 hour. The model explains how the body tries to recover from jet lag by attuning to the presence of various light cues when arriving in different time zones. The finding? Recovery time after traveling east across nine time zones (nearly the equivalent of halfway around the world) is 4 days longer than when traveling west, confirming a suspicion long held by frequent fliers.