In order to monitor the participants’ brains, the researchers

    A new study published in Current Biology is investigating why you get poor sleep in unfamiliar places. It suggests that when people sleep in an unfamiliar place, one hemisphere of the brain stays more awake as a way to keep watch for potential danger possibly a remnant of the days when Homo sapiens had to guard their territory every night.
    This phenomenon is known as unihemispheric slow-wave sleep, and it’s seen in marine animals and some birds. This is the first study to suggest that the human brain may also be hard-wired to function in a similar way, although on a smaller scale. Humans, unlike sparrows, don’t usually sleep with one eye open. However, when in new surroundings, one hemisphere of the brain may stay at least a little bit awake – great for waking quickly if an intruder shows up, but with a resulting groggy feeling the next morning.
    The group of researchers recruited sleep study participants, and conducted neuroimaging along with polysomnography, a standard test used in sleep labs to monitor brain waves, oxygen level in blood, heart rate, breathing, and eye and leg movements. They discovered that only the brain’s right hemisphere was consistently engaged in slow-wave, or deep, sleep. The left hemisphere – the side responsible for logical thinking and reasoning – had what the researchers called “enhanced vigilance”, which also made the entire brain more responsive to sound.
    The researchers tried a test where they targeted sounds to the left and right ear. They found that on the first night, 80 percent of the arousals from deep sleep occurred when sound was made to target the right ear (the brain’s left hemisphere). On day two, that number dropped to about 50 percent.