Researchers say the reason people don’t sleep well in new surroundings is because a part of the brain stays alert for potential dangers.

Frequent travelers know that it’s hard to get truly restful sleep in a new place.

This so-called “first-night effect” is so well-known to sleep researchers that they generally have research participants sleep in the lab for a night before a study begins.

That made Masako Tamaki, Ph.D., a postdoctoral research associate in Cognitive, Linguistic, and Psychological Sciences at Brown University, curious about why people can’t sleep normally in new places.

What Tamaki found was the left hemisphere of the brain doesn’t drop off fully into slow-wave sleep, the deepest sleep stage, on the first night at a new place.

It appears to be an evolutionary adaptation that allows us to stay alert to possible danger.

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The study showed that a part of the left hemisphere’s default-mode network, the brain system that drives slow-wave activity, didn’t drop off as far into sleep as the other parts of that hemisphere on the first night.

Tamaki found the asymmetry was reduced on the second night. Her findings were published today in the journal Current Biology.

The longer it took a person to fall asleep, the more asymmetrical their brain activity was. And the more alert part of the brain was especially sensitive to unfamiliar noises.

Tamaki’s research showed that the part of the brain that remained partially awake was able to rouse sleepers more quickly.

In other words, in new environments where there could be danger, our brains continue to patrol for danger even while we sleep.

“It’s a kind of safeguard of the sleeping brain,” Tamaki told Healthline.

But the trade-off is that we don’t feel as rested in the morning.

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The study looked at the default-mode network because while most connectivity between regions of the brain is broken during sleep it continues to work as a network.

The findings don’t rule out the possibility that other intrinsic networks may also stay partially active at night, particularly the subcortical circuits that help regulate sleep and wakefulness.

The likely evolutionary origin of this capability is much more strongly suggested because sleeping birds and marine mammals also have an imbalance in activity between the two hemispheres of their brains.

The more alert hemisphere allows marine mammals, for example, to surface for air periodically while they sleep.

When Orfeu Buxton, Ph.D., an associate professor in the Department of Biobehavioral Health at Pennsylvania State University, was asked about the findings, his response was, “So we’re dolphins again?”

But perhaps the more pressing issue for most of us is what we can do to sleep better during travel.

“I think you can bring something like your favorite pillow to make the environment as comfortable as possible,” Tamaki said. “But because this system might be related to something fundamental, it might be difficult to completely knock it out.”

The best thing may be to plan to arrive two nights before an important event, she said.

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