Left brain stands guard while sleeping away from home

Away from home, people sleep with one ear open.

In unfamiliar surroundings, part of the left hemisphere keeps watch while the rest of the brain is deeply asleep, scientists report April 21 in Current Biology. The results help explain why the first night in a hotel isn’t always restful.

Some aquatic mammals and birds sleep with half a brain at a time, a trick called unihemispheric sleep. Scientists have believed that humans, however, did not show any such asymmetry in their slumber.

Study coauthor Yuka Sasaki of Brown University in Providence, R.I., and colleagues looked for signs of asymmetry on the first night that young, healthy people came into their sleep lab. Usually, scientists toss the data from the inaugural night because the sleep is so disturbed, Sasaki says. But she and her team thought that some interesting sleep patterns might lurk within that fitful sleep. “It was a little bit of a crazy hunch,” she says, “but we did it anyway.”

During a deep sleep stage known as slow-wave sleep, a network of nerve cells in the left side of the brain showed less sleep-related activity than the corresponding network on the right side. Those results suggest that the left side of the brain is 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 on the second night of sleep.

This more vigilant brain system is the left hemisphere’s default mode network, a collection of nerve cells active when the brain isn’t doing anything in particular. It’s not clear how this network keeps watch during sleep.

The default mode network in the left side of the brain reacted faster to quiet sounds on the first night of sleep, further experiments revealed. Tones played to the right ear, which sends sounds to the brain’s left hemisphere, were more likely to rouse sleeping people than tones played to the left ear. And response times were faster on the first night than the second.

Because the experiment tested only the first slow-wave sleep session of the nights, the scientists don’t know whether the left hemisphere keeps watch all night long. Further experiments are needed to see if the right side ever takes a turn, Sasaki says.

This alertness make sense, says sleep researcher Jerome Siegel of UCLA. “Sleep is only adaptive if it doesn’t produce risks that outweigh its benefits,” he says. And safe sleep often entails keeping an eye on the environment. A more general version of this vigilance probably happens in familiar places, too, says Siegel, pointing to the old saw that a parent can sleep through a thunderstorm but awaken to a baby’s whimpers.