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At home, you might sleep on a bed of straw in a stuffy room that sits directly above the city's newest oontz-oontz factory. But there's a good chance you'd still log better Zzzs in your own bed than you would in a quiet, luxe hotel that you've never been to before. It's not just you: Falling asleep in new environments poses a challenge to insomniacs and sound sleepers alike. And now science has a name for it: It's called the "first-night-effect."

A new study from Brown University, published in Current Biology, explores the neurologic basis of the phenomenon, as Gizmodo reported. As it turns out, the left hemisphere of the brain remains somewhat alert when people spend the night somewhere unfamiliar.

Researchers used neuroimaging to look at brain activity in 35 participants who spent the night in an (unfamiliar) sleep lab. Across the board, one neuronal network, the default-mode network (DMN), exhibited activity in the left side of the brain, but not the right. In particular, the left side tended to light up during slow-wave sleep, when rest is deepest and the brain takes a break from absorbing stimuli (sounds, smells) in the outside world. The left side also showed more sensitivity to sound. Researchers found they could rouse participants more easily when they played beeping sounds in their right ears (because the left side of the brain controls the right side of the body).

When we're sleeping somewhere new, our brains stay vigilant to protect against unknown threats.

Participants returned to the sleep lab for follow-up sessions, but researchers did not see the same asymmetrical brain activity. Once the sleep lab registered as a familiar environment, participants' brains simmered down. 

The DMN tends to take over during introspective thought and mind-wandering. Researchers surmised that deep-sleep DMN activity serves an evolutionary purpose, acting as an alarm system of sorts. When we're sleeping somewhere new, our brains stay vigilant to protect against unknown threats. This might not be entirely necessary today. An Airbnb in an up-and-coming neighborhood doesn't pose much harm. But, thousands of years ago, before the sharing economy really took off, dozing in unknown territory was a little dicier. 

For some species, asymetrical brain activity is the norm during sleep. In dolphins, for example, one hemisphere stays busy so the marine mammals can navigate new waters and come up for air while they doze. It now appears that humans have a limited capacity for this type of "unihemispheric sleep." 

The study affirms what sleep scientists know from running sleep experiments: Discount data from the first night, aka, "adaptation night." Once sleep-study subjects get acquainted with their new environs, their sleep brain waves are ripe for scrutiny.