Half of brain keeps awake during sleep in new places: study

Have trouble sleeping on your first night in a new place? That is because one brain hemisphere remains more awake than the other, apparently in a state of readiness for trouble, a new study has found.

Scientists explain what underlies the "first-night effect," phenomenon that poses an inconvenience to business travellers and sleep researchers. Sleep is often noticeably worse during the first night in, say, a hotel or a sleep lab.

"In Japan they say, 'if you change your pillow, you cannot sleep.' You do not sleep very well in a new place. We all know about it," said Yuka Sasaki from Brown University in the US.

Over the course of three experiments, researchers used several methods to precisely measure brain activity during two nights of slumber, a week apart, among a total of 35 volunteers.

They consistently found that on the first night in the lab, a particular network in the left hemisphere remained more active than in the right hemisphere, specifically during a deep sleep phase known as "slow-wave" sleep.

When researchers stimulated the left hemisphere with irregular beeping sounds (played in the right ear), that prompted a significantly greater likelihood of waking, and faster action upon waking, than if sounds were played in to the left ear to stimulate the right hemisphere.

In other sleep phases and three other networks tested on the first night, there was no difference in alertness or activity in either hemisphere.

On the second night of sleep there was no significant difference between left and right hemispheres even in the "default-mode network" of the left hemisphere, which does make a difference on the first night.

The testing, in other words, pinpointed a first-night-only effect specifically in the default-mode network of the left hemisphere during the slow-wave phase.

For the study, researchers used electroencephalography, magnetoencephelography, and magnetic resonance imaging to make unusually high-resolution and sensitive measurements with wide brain coverage.

The findings were published in the journal Current Biology.