Jet lag affects everyone differently. When you ask your friends about their jet lag, everyone will have a different answer. Some will say it’s worse going eastbound, some will say westbound, and the absolute worst answer to hear is “I don’t get jet lag at all.” Much of this pesky phenomenon depends upon the individual, but now there’s actually a study that claims to have solved the mystery of jet lag once and for all. The Sleep Cycle Institute examined more than 1.5 million nights’ worth of sleep data from people traveling in all directions, across a variety of time zones, and determined that traveling east is, in fact, much worse for sleep than traveling west.

In looking at the first five days of travel, the study showed that people have far less severe jet lag when they travel west. Not only that, but their quantity and quality of sleep, and their wake-up mood, actually improve over those who haven’t even traveled at all.

Interestingly, according to the data, travelers tend to sleep fairly well on the first night of their trip, but sleep quality declines on the second night — not fully returning to normal until day 10.

Dr. Catherine Darley, natural sleep expert, said, “Based on sleep physiology, it makes sense that westward travel is easier, as it is easier to lengthen the circadian rhythm. Not only is mood worse, but people can feel an increase in anger and performance problems. It’s important to note that these effects can last for several days and to take that into account when planning. For those people who travel regularly, there are strategies that can decrease the disruption caused by jet lag.”

Whatever science might say, jet lag remains a highly individual issue. One Matador Network writer, for example — who may or may not be recovering from jet lag at this very moment — would argue that westbound jet lag is why he hasn’t gotten a good night’s sleep in four days.

To each their own.