People who go to bed wary of potential danger sometimes pledge to sleep "with one eye open." A new Brown University study finds that isn't too far off.

On the first night in a new place, the research suggests, one brain hemisphere remains more awake than the other during deep sleep, apparently in a state of readiness for trouble.

The study in Current Biology explains what underlies the "first-night effect," a phenomenon that poses an inconvenience to business travellers and sleep researchers alike.

Sleep is often noticeably worse during the first night in, say, a hotel or a sleep lab. In the latter context, researchers usually have to build an "adaptation night" into their studies to do their experiments. This time around, the team at Brown investigated the first-night effect, rather than factoring it out.

"In Japan they say, 'if you change your pillow, you can't sleep,'" said corresponding author Yuka Sasaki, research associate professor of cognitive linguistic and psychological sciences at Brown. "You don't sleep very well in a new place. We all know about it."

Sasaki and lead author Masako Tamaki wanted to figure out why. Over the course of three experiments, their team used several methods to precisely measure brain activity during two nights of slumber, a week apart, among a total of 35 volunteers.

They consistently found that on the first night in the lab, a particular network in the left hemisphere remained more active than in the right hemisphere, specifically during a deep sleep phase known as "slow-wave" sleep.

"To our best knowledge, regional asymmetric slow-wave activity associated with the first-night effect has never been reported in humans," the authors wrote.

To make the novel findings, the researchers used electroencephalography, magnetoencephelography, and magnetic resonance imaging to make unusually high-resolution and sensitive measurements with wide brain coverage.

Sasaki said it's not known yet why the brain only maintains an alert state in just one hemisphere -- whether it's always the left or in alternation with the right.

There are many examples among animals, however, of hemispheric asymmetry during slow-wave sleep. Marine mammals exhibit it, Sasaki said, presumably because they regularly need to resurface to breathe, even during sleep.

Now it's been found in humans as a first-night phenomenon.

"The present study has demonstrated that when we are in a novel environment, inter-hemispheric asymmetry occurs in regional slow-wave activity, vigilance and responsiveness, as a night watch to protect ourselves," the study concludes.