When we bed down in a new locale, our sleep often suffers. A recent study finds that this so-called first-night effect may be the result of partial wakefulness in one side of the brain—as if the brain is keeping watch.

Researchers at Brown University and the Georgia Institute of Technology used neuroimaging and a brain wave–tracking approach called polysomnography to record activity in four brain networks in 11 individuals as they slept on two nights about a week apart. The subjects nodded off at their normal bedtimes, and their brain was scanned for about two hours—the length of a sleep cycle.

As participants slept, right hemisphere regions showed consistent slow-wave activity regardless of the night. Yet average slow-wave activity was shallower in their left hemisphere during the first night—an asymmetry that was enhanced in those who took longer to fall asleep.

The results, published in May in Current Biology, suggest systems in one side of the brain remain active as people venture into unfamiliar sleep situations—an apparent survival strategy reminiscent of the unihemispheric sleep reported in certain animals.

Because the results represent just one sleep cycle, however, it is unclear whether the left side of the brain is always tasked with maintaining attentiveness, explains the study's senior author Yuka Sasaki, a cognitive, linguistic and psychological sciences researcher at Brown. It is possible the right hemisphere takes over guard dog duties at some point in the night.

Based on anatomical sites with muted slow-wave activity, the researchers suspect the first-night effect involves the default-mode network, a system of interacting brain regions involved in daydreaming and spontaneous thoughts. That network is usually focused inward, though; nighttime watchfulness would be an odd task for it to take, says Massachusetts General Hospital psychiatry researcher Dara Manoach, who was not involved in the study. Still, she says, the differences in left brain activity “link us to the rest of the animal kingdom,” offering an “evolutionarily sensible” scenario that explains the first-night effect.