ASYMMETRIC

When we sleep in new places, half of our brain stays awake

“How did you sleep?” I’ve asked countless friends this question after they’ve spent a night on my (surprisingly comfortable) couch. Turns out, most of them probably lied when they said, “Great!”

Difficulty sleeping in a new environment is so common that neuroscientists have a name for it: the “first-night effect” (FNE). New research shows FNE is basically the neurological equivalent of sleeping with one eye open. When you go to sleep for the first time in a new environment only half of your brain really rests, according to a study recently published in Current Biology.

The researchers tested people sleeping in a new environment by measuring their brainwave activity in the third stage of non-rapid eye movement sleep (NREM 3), which is the deepest stage of the sleep cycle. In their first experiment, the researchers found that sleeping subjects experienced much more activity in the left hemisphere of their brains than in the right hemisphere on the first night of sleep, indicating that the left hemisphere remained relatively alert to the surrounding environment.

When subjects slept in the same place on a second night, the asymmetry went away and both hemispheres rested soundly.

Many birds and aquatic mammals, including dolphins and sea lions, sleep like this all the time. They rest only half of their brains fully, known as unihemispheric slow-wave sleep, so that they can stay alert to potential threats while they sleep. To test whether or not the asymmetric sleep pattern exhibited by humans sleeping in unfamiliar environments serves the same function, the researchers examined whether the lighter sleep characteristic of FNE made the sleeping human subjects more reactive to external stimuli.

To do this, the researchers asked the subjects to tap their fingers when they awoke after hearing sounds while they were sleeping. These subjects slept for two nights in the same place, and the researchers played sounds both nights. The reaction time from a sound to a tap was significantly faster on day one than on day two. This indicates that the brain is not only more perceptive due to FNE, but that it can also actually wake up faster.

The researchers are not sure why FNE causes the left hemisphere of the brain to stay alert rather than the right hemisphere, but they say it may be because the hemispheres are wired differently. The neural connections between the part of the brain in which deep sleep occurs, known as the default-mode network, and the rest of the brain are stronger in the left hemisphere. This may make wakefulness in the left hemisphere more useful than wakefulness in the right hemisphere, as the stronger connections could produce faster responses to stimuli perceived while sleeping.

Even the comfort level of the bed appears not to matter when it comes to sleeping in a new environment. “In our study, we collected a subjective report on discomfort,” says Yuka Sasaki, one of the researchers who conducted the study. “No one really indicated it was uncomfortable on the first session, but everybody shows the FNE.”

Together these results suggest that the light, asymmetric sleep we experience on the first night in a new environment is a protective mechanism in humans, just as it is in animals. But, luckily, it appears that humans are able to return to sleeping deeply once we are familiar with a new location.

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