The old adage about sleeping with one eye open in an unfamiliar place may not be too far off the mark. A new study suggests that one half of the brain remains on high alert during the first night of sleep in a new space.
Over the course of three experiments on 35 young, healthy volunteers, researchers measured brain activity during two consecutive nights of slumber. They consistently found that part of the left side of the brain remained more active than the right side only on the first night, specifically during a deep sleep phase known as slow-wave sleep.
"When you sleep in a new place for the first time, a part of one side of the brain seems to stay awake for surveillance purposes, so you could wake up faster if necessary," said senior study author Yuka Sasaki of Brown University.
While this may be bad news for business travelers who regularly make brief overnight trips, it may not be as troublesome for people who go away for longer periods of time, Sasaki added by email.
"Frequent travel may lead to unrestful sleep," Sasaki said. "But if you stay for a few days at the same place, your sleep might catch up."
To see how being in a strange place impacts sleep, Sasaki and colleagues performed a series of lab tests on their subjects.
When they stimulated the left hemisphere with irregular beeping sounds in the right ear during deep sleep on the first night, that prompted significantly greater likelihood of waking and faster action upon waking, than if sounds were played in the left ear to stimulate the right hemisphere.
In other sleep phases during the first night, and with other tests, there wasn't any difference in alertness or activity between the two hemispheres of the brain, the researchers report in the journal Current Biology.
On the second night, there wasn't any difference in reactions to tests between the left and right hemispheres, even during deep sleep.
This suggests that there is a first-night-only effect specifically in one hemisphere of the brain during deep sleep, the authors conclude. The way participants responded to the sleep lab tests points to the potential for the brain to be on high alert for danger during the first night in unfamiliar surroundings.
Some birds have been found to literally sleep with one eye open and one side of the brain awake when they're in a dangerous setting, and some marine mammals have similar abilities, the authors note.
One limitation of the new study is its focus on healthy volunteers, which means the results may not apply to people with insomnia or other sleep disorders, the authors note.
While it's possible that the findings may explain poor sleep among frequent travelers, the study wasn't designed to test whether these "first night effects" continue to happen to people every time they hit the road, said Patrick Finan, a psychiatry and behavioral health researcher at Johns Hopkins University School of Medicine in Baltimore.
"It is possible, for example, that frequent travelers might adapt to this first night effect over time," Finan, who wasn't involved in the study, said by email.
"Any clinical implications would be speculative at the moment," Finan added. "However, the level of specificity provided by these analyses could be an important first step in understanding who might be at risk for sleep disorders like insomnia, which is thought to be driven in many patients by chronic hypervigilance."