The left hemisphere of our brain stays alert when we sleep in an unusual place. It takes a night to get used to.
The Japanese say “change your pillow, and you won’t sleep.” We have all experienced it, at one time or another: in an unfamiliar environment (at a hotel, with friends, in a vacation home), the first night is often at least partially white. This is due to the left hemisphere of our brain, according to researchers at Brown University (United States). Their study, published in Current Biology, reveals that the left hemisphere remains alert to external stimuli, ready to wake us up at any time to face possible danger.
This difficulty in sleeping the first night out of the usual bed is well known to sleep specialists, who call it the “first night effect”. When they conduct an experiment, the results are only taken into account after this night of adaptation. For this study, Brown’s researchers instead observed the brain activity of 35 people using the combination of an electroencephalograph, an MRI and a magnetoencephalograph.
Sleep with one eye
By the second night, nocturnal brain activity was normal. But for the first, the researchers observed a particular activity of the left hemisphere. As the subjects of the experiment slept soundly, small sounds were emitted to one ear or the other. When sound came from the right ear – connected to the left hemisphere where areas were activated – sleepers tended to wake up and be active much faster.
The Brown University team showed that the left hemisphere remained awake during the first phase of deep sleep, but did not study the rest of the night. “It is possible that for surveillance, the hemispheres alternate,” suggests Yuka Sasaki, a cognitive linguistics and psychology researcher at Brown University.
Scientists are leaving the door open to the idea of a functioning similar to that which we know in marine mammals such as dolphins. They only put one hemisphere of their brain to sleep at a time, in turn, so they can continue to rise to the surface and breathe.
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