Why people sleep badly in an unfamiliar place
People sleep less well in an unfamiliar place as the brain's left side stays alert for danger, a study suggests.
Images of volunteers' brains during nights spent in the lab showed the left side remained more responsive to sound.
This asymmetry, also found in marine animals and some birds, was observed only on the first night in a new place.
And Yuka Sasaki, from Brown University, Rhode Island, said it was possible people could learn to switch off their "nightwatchman" function.
"Humans' brain are very flexible," she wrote in the journal Current Biology.
"Thus, people who are often in new places may not necessarily have poor sleep on a regular basis."
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