It’s quite common to have trouble getting a good night’s sleep in a new environment — and, as a result, to feel tired the next day — even if the new setting is comfortable and quiet.
In fact, sleep researchers have given the phenomenon a name — the “first-night effect” — and it’s why they usually ignore data collected during the initial night a person sleeps in a lab for a sleep-related study.
The reason for the first-night effect has been a puzzle, however — until, perhaps, now. A new study has found that when we’re in an unfamiliar place, such as a hotel room, one hemisphere of our brain tends to stay vigilant as we sleep.
It’s apparently acting like a sentry, trying to keep watch.
“We know that marine animals and some birds show unihemispheric sleep, one awake and the other asleep,” said Yuka Sasaki, one of the study’s authors and an associate professor of cognitive, linguistic and psychological sciences at Brown University, in a released statement.
Some of these animals, like dolphins, even keep the eye that corresponds with the “awake” hemisphere open while they sleep.
Human brains are not that asymmetric, but the new findings suggest that “our brains may have a miniature system of what whales and dolphins have,” said Sasaki.
The study was published Thursday in the journal Current Biology.
A trio of experiments
For the study, which involved three different experiments, Sasaki and her colleagues recruited 35 volunteers to sleep in her lab for two nights, a week apart. While they slept, their brain waves, heart rate, blood oxygen levels, eye and leg movements, and other body processes were carefully measured and recorded by various devices.
As expected, the participants took longer to fall asleep on their initial night in the lab than on the subsequent one. They also slept less deeply.
What wasn’t expected was something that appeared on the neuroimages: During the inaugural night in the lab — and only during that night — the two hemispheres of the brain displayed different patterns of activity. The left half showed significantly less activity than the right half during the first slow-wave phase of the night.
Slow-wave brain activity is associated with deep sleep.
This finding suggests, Sasaki and her co-authors write in their paper, that a neural network in the left hemisphere works “as a night watch in an unfamiliar environment to protect the sleeper.”
Interestingly, the volunteers who had the most difficulty falling asleep on the first night also exhibited the greatest differences in slow-wave activity between their two brain hemispheres.
Sounding out the difference
To test if the left hemisphere was actually being more vigilant, Sasaki and her colleagues had volunteers wear headphones while they slept. Different patterns of beeps were sent through the devices — sometimes to the right ear, and sometimes to the left. (Our ears send signals to the hemisphere that is opposite them.)
The researchers found that the left hemisphere showed a greater response than the right to the sounds. This difference occurred, however, only when the beeps were irregular and high-pitched — a pattern meant to mimic something unusual and, thus, potentially dangerous.
As in the first experiment, the difference disappeared after the first night.
In their third experiment, Sasaki and her colleagues played a sound designed to wake a light sleeper through the headphones. The participants woke up quicker when the sound was played through their right ear, which sends messages to the left side of the brain.
The researchers don’t know why the left hemisphere serves as the “watchman,” but they suggest that its vigilance is an evolutionary adaption to help protect us from unknown dangers in unfamiliar settings.
Bring a pillow
This study had several important limitations. Most notably, it involved a small number of people and monitored only the first phase of slow-wave (deep) sleep. Humans experience four or five deep-sleep phases during a typical night. It could be that the role of “watchman” shifts between the two brain hemispheres during a later sleep cycle.
Sasaki and her colleagues intend to explore that possibility in future research. In the meantime, travelers who want to turn their brain’s night vigilante off might try bringing their own pillow, said Sasaki. Or they could try booking in the same hotel chain, whose rooms will look familiar and therefore might not elicit a first-night effect from the brain.
Some very frequent travelers, however, do not complain of feeling sluggish after sleeping in new surroundings. It could be that their brains have learned to shut down the left hemisphere’s watchman, said Sasaki.
“Human’s brains are very flexible,” she added. “Thus, people who often are in new places may not necessarily have poor sleep on a regular basis.”
For more information: You can read the study in full on the Current Biology website.