People sleep with one ear open when they’re away from home.
The brain is divided into two halves, known as the left and right hemispheres. In unfamiliar surroundings, part of the left half keeps watch while the rest of the brain sleeps deeply. That helps explain why the first night of a sleepover at a friend’s home or at some vacation spot may not be restful.
Birds and some aquatic mammals always send only half their brain to sleep at a time. This trick is called unihemispheric (Yoo-nee-hem-iss-FEER-ic) sleep. But scientists had thought people had no such lopsidedness in their slumber.
Although the brain rests during sleep, it doesn’t totally shut down. So researchers measured brain activity during sleep. For the new study, those scientists recruited young, healthy people to spend a night in a sleep lab.
The idea that people don’t sleep well on the first night in an unusual place is not new. In fact, scientists usually toss out data from the initial night in a sleep lab because it tends to be so disturbed, says Yuka Sasaki. This psychologist at Brown University in Providence, R.I., co-authored the new study.
Sasaki and her team suspected there might be interesting trends in sleep patterns during that first night of fitful sleep. “It was a little bit of a crazy hunch,” she recalls, “but we did it anyway.”
One stage of deep slumber is known as slow-wave sleep. In this stage, a group of nerve cells in the left side of the brain showed less sleep-related activity than the same group on the right side. That suggests the left side of the brain was a lighter sleeper.
“It looked like the left hemisphere and the right hemisphere did not show the same degree of sleep,” Sasaki says. This imbalance disappeared by the second night. Sasaki and her colleagues published their results April 21 in Current Biology.
Both the right and left sides of the brain have something called a “default mode network.” It’s a collection of nerve cells that are active when the brain isn’t focused on doing anything special. And only the default mode network of the left hemisphere remained vigilant during sleep.
The researchers played tones into the ears of each sleeping volunteer. They found that the default mode network in the left side of the brain reacted faster to quiet sounds on the first night of sleep. Tones played into just the right ear — which sends sounds to the brain’s left hemisphere — were more likely to wake a sleeper than were tones played into the left ear. And a sleeper’s response times were faster on the first night than on the second.
On a normal night, the brain goes through a few slow-wave sleep cycles. But the experiment only tested the first slow-wave sleep session of a night. So the scientists don’t know whether the left hemisphere keeps watch all night long. Sasaki says that more experiments are needed to see if the right side ever takes its turn.
This alertness makes sense, says Jerome Siegel. He studies sleep at the University of California, Los Angeles. “Sleep is only adaptive if it doesn’t produce risks that outweigh its benefits,” he says. And safe sleep often means keeping an eye on the environment.
A more general version of this vigilance probably happens in familiar places, too, Siegel says. He points to the old observation that a parent can sleep right through a thunderstorm but wake up in response to a baby’s whimpers.
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adaptation A process by which an organism or species becomes better suited to its environment. When a community of organisms does this over time, scientists refer to the change as evolution. Adaptive is an adjective that describes some change that leaves an organism or species better suited to its environment.
aquatic An adjective that refers to water.
asymmetry Not symmetrical, such as not the same shape on the left and right sides.
birds Warm-blooded animals with wings that first showed up during the time of the dinosaurs. Birds are jacketed in feathers and produce young from the eggs they deposit in some sort of nest. Most birds fly, but throughout history there have been the occasional species that don’t.
default mode network An area of the brain where nerve cells rev up their activity at those times when someone is not focused on a specific task. These cells work behind the scenes when someone is daydreaming, sleeping or otherwise at rest.
hemisphere It means half of a globe, and can refer to half of the Earth, half of some ball, or anything else, such as half of the brain.
mammal A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding the young, and (typically) the bearing of live young.
neuron or nerve cell Any of the impulse-conducting cells that make up the brain, spinal column and nervous system. These specialized cells transmit information to other neurons in the form of electrical signals.
slow-wave sleep Also known as deep sleep or non-REM sleep. This is the largely dreamless stage of sleep when a person or animal gets the most rest.