When you sleep in unfamiliar surroundings, only half your brain is getting a good night's rest.
"The left side seems to be more awake than the right side," says Yuka Sasaki, an associate professor of cognitive, linguistic and psychological sciences at Brown University.
The finding, reported Thursday in the journal Current Biology, helps explain why people tend to feel tired after sleeping in a new place. And it suggests people have something in common with birds and sea mammals, which frequently put half their brain to sleep while the other half remains on guard.
Sleep researchers discovered the "first-night effect" decades ago, when they began studying people in sleep labs. The first night in a lab, a person's sleep is usually so bad that researchers simply toss out any data they collect.
But Sasaki wanted to know what was going on in the brain during that first night. So she and a team of researchers studied the brain wave patterns of 35 Brown University students.
The team measured something called slow-wave activity, which appears during deep sleep. And they found that during a student's first night in the lab, slow wave activity was greater in certain areas of the right hemisphere than in the corresponding areas of the left hemisphere.
After the first night, though, the difference went away.
To confirm that the left side of the brain really was more alert, the team did two other experiments. First, they had the sleeping students listen to a repeated standard tone followed by a single tone of a different pitch.
When someone is awake or sleeping lightly, the brain responds to this "deviant tone." And the students' brains did respond — but only on the left side.
Then the researchers played a sound loud enough to wake someone who was sleeping lightly. And they found that students woke up faster when the sound was played into the right ear, which is connected to the left side of the brain.
The ability to rest just one side of the brain has never been demonstrated in people before, says Niels Rattenborg, leader of the avian sleep group at the Max Planck Institute for Ornithology in Seewiesen, Germany. But he says it's a trick many animals can do.
"We've known for quite a while that some marine mammals like dolphins and some of the seals as well as many birds can sleep with one half of the brain at a time," he says.
A few years ago, Rattenborg did an experiment with ducks that suggests at least one way in which half-brain sleeping provided an evolutionary advantage. The experiment involved putting ducks in a row, literally, and watching them sleep.
Rattenborg found that ducks with a bird on either side of them put their entire brain to sleep and kept both eyes closed. "However, the ducks at the end of the row slept more with one half of the brain at a time," he says. "And when they did that they directed the open eye away from the other birds, as if they were looking for approaching predators."
Predators aren't a big problem for people these days. But the human brain was shaped during a time when nights were dark and full of terrors, Rattenborg says.
"When we're sleeping in a new environment and we don't know how many predators are around," he says, "it would make sense to keep half the brain more alert and more responsive to bumps in the night."
Sasaki says that brain response is involuntary and there's nothing people can do to prevent it, even if they've just flown in for a big presentation the next morning. So lots of coffee the next morning.
KELLY MCEVERS, HOST:
It's hard to get a good night's sleep in an unfamiliar place. This problem is so common, it has a scientific name - the first-night effect. And now researchers appear to have figured out what causes it. NPR's Jon Hamilton reports.
JON HAMILTON, BYLINE: When scientists began setting people in sleep labs several decades ago, they noticed something. Almost everyone slept poorly the first night. Yuka Sasaki, a sleep researcher at Brown University, says she found that surprising.
YUKA SASAKI: Even the healthy people cannot sleep. What's going on? So we decided to look into it.
HAMILTON: By studying several dozen Brown University students as they slept in a sleep lab. this a good sleep. Sasaki's team measured something called slow-wave activity in the brain. Slow waves appear during deep sleep. But she says during a student's first night in the lab, the slow waves were concentrated in just one side of the brain.
SASAKI: Part of the brain in the left side seems to be more awake than the corresponding part in the right side.
HAMILTON: After the first night, though, the difference went away. So the team did another experiment. Sasaki says they played a series of tones for students spending their first night in the lab.
SASAKI: We gave sounds (imitating tones) - something like that.
HAMILTON: Brains respond to the higher (imitating tone)-tone when they are awake, and the students' sleeping brains did, too, but only on the left side. Finally, the researchers played a sound loud enough to wake someone sleeping lightly, and they found that students woke up faster when the sound was played into the right ear, which is connected to the left side of the brain.
This is cutting-edge stuff for people who study human brains, but it turns out scientists who study animal brains know all about it. Niels Rattenborg is from the Max Planck Institute for Ornithology in Germany.
NIELS RATTENBORG: Yeah, we've known for quite a while that some marine mammals like dolphins and some of the seals as well as many birds can sleep with one half of the brain at a time.
HAMILTON: A few years ago, Rattenborg did an experiment with ducks that suggests why this ability evolved.
RATTENBORG: We literally put our ducks in a row.
HAMILTON: And watched them sleep. Rattenborg says ducks with a bird on either side of them slept with both halves of their brain and kept both eyes closed.
RATTENBORG: However, the ducks at the end of the row slept more with one half of the brain at a time, and when they did that, they directed the open eye away from the other birds as if they were looking for approaching predators.
HAMILTON: Predators aren't a big problem for people these days, but Rattenborg says the human brain still contains circuits from a time when the nights were dark and full of terrors.
RATTENBORG: When we're sleeping out in the wild and we're in a new environment and we don't know, you know, how many predators are around, it would make sense to keep half of the brain more alert and responsive to bumps in the night.
HAMILTON: Even if that means feeling a bit groggy the next morning. The new research appears in the journal Current Biology. Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.