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.
Scientists 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.
Author 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.