NPR: Half Your Brain Stands Guard When Sleeping In A New Place.

“When you sleep in unfamiliar surroundings, only half your brain is getting a good night’s rest.
[…]
The finding, reported Thursday in the journal Current Biology, helps explain why people tend to feel tired after sleeping in a new place. And it suggests people have something in common with birds and sea mammals, which frequently put half their brain to sleep while the other half remains on guard.”