Jet lag is an actual disorder that happens when your circadian rhythm — which signals you to go to bed or wake up — is still synced up to a time zone you’re no longer in. You must cross multiple time zones over a short period of time to have it. Jet lag can leave you feeling fried in the mornings, and wired at night when you’re trying to fall asleep in a new part of the world (or when you back home from a trip). Symptoms include exhaustion, headaches, nausea, and changes in bowel movements. The more times zones you cross, the worse off you’ll be, although Dr. Dasgupta says it feels easier when you're travelling East to West. Meanwhile, travel fatigue can happen to you even if you’re not changing time zones. Travelling is exhausting, and you might be losing extra sleep going to the airport, or indulging in the nightlife of a new city. Exploring will tucker you out — but it might not actually be disrupting your body’s clock.