If you find it hard to doze off on your first night in unfamiliar surroundings, you're not alone—and it may be because you're like a dolphin. In a study published in Current Biology, Brown University scientists found this type of sleep disturbance (referred to as the "first-night effect") could be because the left side of the brain stays more alert than the right side, an evolutionary throwback that may have allowed our ancestors to sleep while still watching out for predators, NPR reports. "Our brains may have a miniature system of what whales and dolphins have," says co-author Yuka Sasaki, per the Telegraph, with the Atlantic noting that some birds also have this ability to keep one half of their brain alert (and even an eye open) while asleep. The scientists studied the brain-wave patterns of 35 volunteers in a sleep lab, on two separate occasions a week apart, playing sounds in sleeping students' ears.
On the first night, slow-wave activity—indicating deep sleep—was lower in the left side of the brain, meaning it was likely more alert. On the second, the two sides were more closely matched. Scientists cited in the Atlantic seem skeptical, noting the small sample size and the trial's failure to account for factors such as gender. Plus, there are outliers: Insomniacs, for example, link their own bedrooms to their sleep grief, so they often sleep better when away from home, says a King’s College London researcher, per New Scientist. Sasaki notes some people, like those who travel often, may actually "train" their brains to turn night-guard mode off. In the meantime, researchers say there's not much the rest of us can do. "If you need to be well-rested for an event, think about arriving two nights early," co-author Masako Tamaki says. (The CDC has named our most sleep-deprived group.)