A new study offers insight into why you might have a hard time sleeping on the first night in a new place: half of your brain stays awake to watch out for potential dangers.

Image credits Jacob Stewart / flickr

If you’re anything like me you know that feeling you get when sleeping in a new place — it’s not necessarily (but often) a restless night, and even if you do get some sleep it just feels somehow off. If you keep in mind that our brains are still wired to keep us alive in the wild, there’s a pretty straightforward explanation, a new study finds.

Yuka Sasaki of Brown University and her team recruited 35 people to spend several nights in a sleep lab, while they monitored their brain activity with advanced imaging techniques. The results consistently show that on the first night in the lab, subjects’ left hemispheres remained more active during deep sleep (or “slow-wave sleep”) than the right ones. This effect is similar to that seen in marine mammals, that only shut down one half of their brain at a time during sleep, but much less pronounced.

“We know that marine animals and some birds show unihemispheric sleep, one awake and the other asleep,” says Yuka Sasaki of Brown University. “[The findings suggests that] our brains may have a miniature system of what whales and dolphins have.”

The so-called first night effect seems to be caused by the difference in brain activity between the two hemispheres — the more pronounced this difference, the harder it was for participants to fall asleep. The hemisphere with increased activity also showed greater response to sounds — the team found that playing beeping noises in the right ear (corresponding to the left hemisphere) woke participants up more easily than when played in the left ear.

Those asymmetries observed during the first night of sleep weren’t evident in subsequent sleep sessions. All this suggests that our brains delegate the left hemisphere to retain part of its activity when we’re sleeping in a new environment. This way it can serve as a “night watch” that wakes the sleeper up if there’s danger, the researchers said.

But there are things you can do to improve your sleep: just lull your brain into a sense of safety. Bring your pillow along or find a accommodation similar to your usual sleeping spot.

“Human’s brains are very flexible,” Sasaki says. “Thus, people who often are in new places may not necessarily have poor sleep on a regular basis.”

The researchers are planning to test if this first night effect can be reduced or eliminated by lowering brain activity throung transcranial magnetic stimulation.

The full paper, titled “Night Watch in One Brain Hemisphere during Sleep Associated with the First-Night Effect in Humans” has been published online in the journal Current Biology and can be read here.