When people sleep in an unfamiliar place for the first time--a hotel room, for example--they often feel as though they haven't slept as well. Now, researchers reporting in the Cell Press journal Current Biology on April 21 have discovered the reason why: under those conditions, one hemisphere of the brain stays more awake to keep watch.
"We know that marine animals and some birds show unihemispheric sleep, one awake and the other asleep," says Yuka Sasaki of Brown University. While the human brain doesn't show the same degree of asymmetry that the brains of marine animals do, the new findings suggest that "our brains may have a miniature system of what whales and dolphins have."
Researchers have long recognized that people sleep poorly the first night in a new location, a phenomenon known as the first-night effect. As a result, sleep scientists typically throw out data from the first night a person sleeps in the lab, analyzing data from the second sleep session on.
Sasaki and her colleagues, including Masako Tamaki, Ji Won Bang, and Takeo Watanabe, wanted to know why that bad sleep happens. To find out, they used advanced neuroimaging techniques to analyze the sleeping brain.
Those images revealed something they hadn't expected to see: during the first night of sleep, the two hemispheres of the brain showed different patterns of activity. One side of the brain slept more lightly than the other. For reasons the researchers don't yet understand, the more awake part of the brain was always the left side.
The degree of asymmetry observed in those brain patterns was related to the difficulty a person experienced in falling asleep, a critical measure in the first-night effect. Importantly, the hemisphere with reduced sleep depth also showed greater response to sounds. Those asymmetries observed during the first night of sleep weren't evident in subsequent sleep sessions.
Sasaki says people might be able to reduce this effect by bringing their own pillow or staying in hotels with similar accommodations. It's also possible that people who have to sleep in new places often learn to turn this night surveillance off.
"Human's brains are very flexible," she says. "Thus, people who often are in new places may not necessarily have poor sleep on a regular basis."
She says their lab is now trying to temporarily knock out the "awake" part of the brain by transcranial magnetic stimulation to see whether sleep improves.
This work was supported by the National Institutes of Health and the National Science Foundation.
Current Biology, Tamaki et al.: "Night Watch in One Brain Hemisphere during Sleep Associated with the First-Night Effect in Humans" http://www.cell.com/current-biology/fulltext/S0960-9822(16)30174-9
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