(TSOOK-uhn-roo-uh)
(n.) Migratory restlessness in birds

This German word has been adopted into English as an ornithological term for the restless displayed at certain times of year by caged migratory birds. But surely it deserves wider metaphorical use as a word for an "instinctive restlessness" in humans as well.

"Yes, we're back from Europe, but the Zugunruhe is already kicking in."