You know how you never sleep well the first night in a hotel? You toss and turn. Noises startle you easily. It’s a pain in the neck.

Researchers curious about this phenomenon took brain scans of people attempting to sleep in a new place and discovered something curious. In the first few hours of trying to fall asleep, the left hemisphere of the default mode network (DMN), the seat of our internal monologue, maintained a relatively high level of activity. It “stayed up” and acted like a night watchman. It noted noises and bumps in the night more than the rest of the brain. It roused the person over small disturbances, at great frequency, creating the classic tossing-and-turning effect.

Luckily, on the second night of the study,........

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