We’ve all been there—that moment when we get back from a trip and instantly feel like we need a vacation after our vacation. Scientifically, there’s a simple explanation: On long trips, people’s overall health and well-being don’t actually peak until the eighth day of vacation, which says a lot about the straight-up exhaustion and stress that come with travel. Obviously, the goal is to break from routine (don’t get us wrong, you definitely want to take those vacation days), but when you’re thrown off your normal circadian rhythm, regular daily diet, and so on, your body’s working a little bit harder to keep you in good health. Jet lag, colds, and digestion issues like diarrhea are so common, they’re practically expected. But what might catch you by surprise? The tingle of an oncoming cold sore.