Half Of Your Brain Stays Awake When You Sleep In An Unfamiliar Place

When sleeping in a new place, half of our brain is awake and alert while the other half rests.

Have you ever felt tired, restless or groggy after what you thought was a full night’s rest in a new place? Or perhaps jumped out of a hotel bed fully-awake at the slightest disturbance?  A new study explains that this might be because your brain just pulled an all-nighter of guard duty — half of your brain that is.

The phenomenon is called interhemispheric asymmetry and it was first observed by scientists studying the first-night effect (FNE) in the sleeping patterns of  11 people over the course of two nights. FNE is something we have all experienced, when we have troubling sleeping well in a novel or unfamiliar environment, and until now, scientists have long considered this a normal and typical sleep disturbance.

What the scientists from Brown University discovered is that in a new environment, our brains are actively switched into survival mode. Only one hemisphere of the brain is turned off and resting, while the other half stays awake and vigilant. Lead author and sleep scientist Masako Tamaki summarized their findings last year in the science journal Current Biology:

“Troubled sleep in an unfamiliar environment is an act for survival over an unfamiliar and potentially dangerous environment by keeping one hemisphere partially more vigilant than the other hemisphere as a night watch, which wakes the sleeper up when unfamiliar external signals are detected.”

Asymmetrical Activity During Sleep

Tamaki and her colleagues used neuroimaging and a brain wave–tracking approach called polysomnography to observe and analyze brain activity while the subjects slept. This is when they first noticed that one hemisphere of the brain, though not fully awake in terms of normal daytime levels of activity, did indeed “hum” while the other hemisphere slept.

This creates an asymmetrical pattern of sleep activity because one side of the brain does not match the other side. The scientists then observed that brains returned to symmetrical patterns on the second night, after the subjects had become comfortable and familiar with the stimuli in their new environment. They also noticed that it took subjects longer to fall asleep if they had more pronounced asymmetrical patterns.

In the report, Tamaki lists the highlights of their discovery:

  • Interhemispheric asymmetry in sleep occurs for the first night in a new place
  • This interhemispheric asymmetry occurs in the default-mode network
  • One brain hemisphere may work as a night watch during sleep in a novel environment
  • The less-asleep (night watch) hemisphere shows increased vigilance in response to deviant stimuli

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