Whether you’re staying in a hotel or having a sleepover, you never sleep quite as well on a bed that’s not your own.
That’s an observable fact. When scientists have people sleep in a lab for an experiment, they often toss out the first night of data because people sleep so poorly. But before now, they haven’t known why.
In a small new study published in Current Biology, researchers from Brown University found out what goes on in the brain when a person sleeps in an unfamiliar place. They measured brain activity during the deep sleep of 35 young, healthy people.
The researchers found evidence that something unique indeed goes on in the brain during the first night: one hemisphere of the brain, the left, shows wakefulness while the other shows sleep. This alertness during sleep in half of the brain has been observed in other animals—including whales, dolphins and birds—and is thought to act as a kind of night watch. “The environment is so new to us, we might need a surveillance system so we can monitor the surroundings and we can detect anything unusual,” says Masako Tamaki, one of the authors of the study and research associate at the Laboratory for Cognitive and Perceptual Learning at Brown University. We’re most vulnerable when we’re asleep, in other words, and by staying partially awake, our brains might be trying to protect us.
The researchers also found that when they outfitted the people in the study with earphones, the left side showed a larger brain response to high-pitched sounds than the right—suggesting more vigilance in that hemisphere.
The study raises a lot of unanswered questions; researchers don’t yet know why they saw this effect in the left hemisphere and not the right. But interestingly, both of these asymmetries only occurred on the first night—something to keep in mind the next time you can’t fall asleep in a strange place.
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