How half of our brain stays alert once we sleep in unfamiliar places.

Have you ever arrived at a hotel room after an extended flight and located it difficult to go to sleep despite being exhausted? And even when you've managed to go to sleep, do you continue to often get up at night, or get up early within the morning, feeling restless and frustrated?

Researchers have long known about this phenomenon in experimental settings, calling it the “first night effect.” Sleep study participants often sleep poorly during their first experimental session in a brand new environment, and sleep quality normally improves dramatically by the second night.

So what happens within the mind when people sleep in a brand new place? In our study, published today Current biologywe found that disturbed sleep in an unfamiliar environment could also be linked to a vital brain function to guard the sleeper from potential danger.

The effect of the primary night

Studies have indicated unilateral hemispheric sleep. some birds And Marine mammalswhere one hemisphere of the brain sleeps while the opposite is awake.

This trait has been linked to survival strategies. Some birds show unilateral sleep in dangerous situations, e.g When they sleep on the dangerous edge of a group. as a substitute of in the center, in order that the awake hemisphere can detect predators while the opposite half rests.

We hypothesized that something similar might occur within the human brain throughout the first night's effect. Perhaps when people don't know whether a brand new place is secure or not, an internal surveillance system kicks in.

So we tested whether one cerebral hemisphere sleeps lighter – also often known as interhemispheric asymmetry – during an individual's first night in a brand new place.

First experience

The human brain is split into two hemispheres, left and right. Parts of the left hemisphere are related to language processing and parts of the fitting are related to processing spatial information, or processing the encompassing environment.

We used a sophisticated neuroimaging technique to detect the depth of sleep within the cerebral hemispheres of 35 young, healthy participants over two nights, roughly one week apart. Sleep sessions weren’t sequential in order that any effects from the primary session wouldn’t carry over to the second.

The technique involves combined magnetoencephalography (MEG), which measures changes within the brain's magnetic field. Magnetic resonance imaging (MRI), which measures structural brain information. and polysomnography, which measures the final state of sleep.

The technique measured slow wave activity that indicates the depth of sleep. When slow wave activity is robust, sleep is deep. When it’s weak, sleep is light. We found that the left hemisphere of the participants' brains was lighter than the fitting on the primary night.

The depth of sleep in several brain networks was also measured. Brain networks are made up of many various brain regions but they work together. One of them—the default mode network—is related to spontaneous mind wandering. So if one's mind wanders spontaneously, the default mode network might be activated.

We found that it was the default mode network that sleeps less when the left hemisphere is lighter, suggesting the brain is wandering – or on some kind of alert.

We also found that participants with stronger interhemispheric asymmetry within the default mode network slept worse. Interhemispheric asymmetry within the sleeping brain was observed only on the primary night when the environment was novel. During the following session, everyone slept well.

Did the left hemisphere sleep calmly throughout the first session since it was monitoring the environment? If so, this hemisphere would also have the opportunity to react to subtle cues. We tested this possibility in the following experiment.

Next experiment

While the participants were sleeping, they were presented with two different beeps through earphones. One was a loud, abnormal frequency sound, while the opposite was a standard frequency sound.

Most of the time, participants heard a standard sound, but occasionally, we presented them with a rare sound. We then measured how much each hemisphere responded to the 2 sounds.

We found that the light-sleeping (left) golard was more alert than the fitting when presented with an unusual sound. He actually responded strongly to unusual sounds but not so strongly to normal sounds. Again, these effects were only observed throughout the first sleep session and never the following.

We then wondered whether this vigilance meant that individuals could get up and react quickly to unusual signals in a brand new place. We asked participants to tap their fingers once they heard a sound. As within the previous experiment, beeps were presented through earphones while the participants were asleep.

In the primary session, participants stood up and tapped their fingers faster to an unfamiliar sound than to a different, after the room had been familiarized. And these were linked to the left hemisphere that detects sounds.

We also noted that participants' anxiety levels didn’t differ between the 2 sleep sessions, so that is unlikely to be an element.

As in some animals, the interhemispheric brain balance that happens throughout the first night in humans may act as a protective buffer to guard them from danger.