A new study shows that half of the brain remains alert when a person sleeps in a new location, which may explain why a person does not sleep well in a new place.

Scientists have long known that results from the first night of most sleep experiments are usually a bit off. There’s even a science-y name for this called “first-night effect” or FNE. The FNE results are so atypical, some researchers will toss them out. Wanting to understand why this happened, scientists at Brown University devised an unusual experiment.

They wired people up to brain-monitoring equipment and played quiet and infrequent beeps by each ear of the sleeper. Researchers found that on that first night of the experiment the beeping on the left side of the brain reacted strongly to the sound, compared to the right side. The left side is related to thinking needed for a kind of vigilance. Noises played on the left side more often woke people up. On the second night of the experiment the night watchman camped out in the left side of their brain seemed to be asleep on the job. Both brain hemispheres responded at the same level and the beeps woke fewer people up.

What that suggests is that humans may be a bit bird-brained. Birds can actually switch off half their brain when they sleep. By literally keeping one eye open, that eye sends information to the side of their brain that corresponds with it while awake. So even while still asleep, the awake side of the brain can make decisions to fly or fight and help protect them from a hungry cat or an aggressive hawk.

Birds can even rotate which side of their brain stays awake depending on where they are sleeping. Like birds, our brains have two hemispheres, but when we see something our eyes send that information to both side of the brain. Our brains are joined together by a tiny bundle of nerves, unlike birds. Something about that unfamiliar environment must be keeping that left side of our brain awake, even though when we don’t sleep with one eye open.

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