It’s summer travel season, and there’s nothing quite like the anticipation you feel when you board a plane and cross time zones en route to your vacation destination. But there’s also nothing quite as awful as that sleepy, hungover feeling you end up with when jet lag throws your body clock totally out of whack. “When we travel across time zones, we expose our internal biological (or circadian) clock to a different light-dark cycle,” says Charmane Eastman, founding director the Biological Rhythms Research Lab at Rush University in Chicago.