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How deep do you sleep? Israeli scientists claim to have the answer

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Prof. Yuval Nir’s lab at Tel Aviv University’s Sackler Faculty of Medicine and Sagol School of Neuroscience looked into the reasons why, during sleep, we rarely respond to external stimuli such as sounds

By ILANIT CHERNICK

Sleep is a key component for a healthy lifestyle, but what is it that keeps us asleep? What stops us from being woken up by sounds around us?

Prof. Yuval Nir’s lab at Tel Aviv University’s Sackler Faculty of Medicine and Sagol School of Neuroscience explored just that. 

In a study released on Monday, Nir and his team looked into the reasons why, during sleep, we rarely respond to external stimuli such as sounds even though our brains remain highly active. 

The team discovered that there is “a tiny region in the brainstem called the locus coeruleus,” which secretes a chemical called noradrenaline throughout the brain, and it’s this that plays a crucial role in how deeply we sleep. 

According to the study, noradrenaline, which is secreted in response to stress, “lies at the heart of our ability to ‘shut off’ our sensory responses and sleep soundly.”

Led by TAU doctoral student Hanna Hayat of Prof. Nir’s lab, the research was conducted in collaboration with Prof. Tony Pickering of Bristol University, Prof. Ofer Yizhar of the Weizmann Institute of Science and Prof. Eric Kremer of the University of Montpellier. 

“The ability to disconnect from the environment, in a reversible way, is a central feature of sleep,” Hayat explained, pointing out that when we are experiencing a good, deep sleep “we rarely respond to external stimuli such as sounds, unless they are strong enough, like an alarm clock, or meaningful, like a baby’s cry, which wake us up.” 

The need and ability to sleep through sounds and external stimuli “is critical for our health and well-being.” 

But, the team stressed, “we differ in how easily we wake up: Light sleepers wake up from every faint sound, while deep sleepers can sleep through just about anything.

“We all can have dramatic changes in our sleep quality during periods of stress,” they added.

How do we stay disconnected from the world when we sleep? (Credit: Ana Yael)
How do we stay disconnected from the world when we sleep? (Credit: Ana Yael)

Hayat explained that the more activity there is in this brain system, “the more likely we are to wake up from a sound. 

“The normal healthy situation is for noradrenaline activity to be silent or minimal during sleep, but when noradrenaline activity is high, we wake up more frequently, even from low-volume sounds,” she said. “This discovery can explain the changes in sleep quality between different people.”

Discussing the method of their research, the scientists used rat models to determine the level of locus coeruleus activity during sleep and which sounds, if any, would be responsible for waking up the rodents. 

They found that the rats’ varying levels of locus coeruleus activity accurately predict if the animals would awaken in response to sounds. The team then silenced locus coeruleus activity through optogenetics. This harnesses light and controls neuronal activity. 

After putting this into play, the scientists discovered that the rats did not easily wake up in response to sound.

Hayat pointed out that when the team increased the noradrenaline activity of the locus coeruleus while a sound played in the background, “the rats woke up more frequently in response, but when we decreased the activity of the locus coeruleus and played the same sound in the background, the rats rarely woke up. 

“So, we can say that we identified a powerful ‘dial’ that controls the depth of sleep despite external stimuli,” she added.

Hayat concluded that their “findings clearly show that the locus coeruleus noradrenaline system plays a crucial role in this disconnection by keeping a very low level of activity during sleep.”

Commenting on the findings, Nir highlighted that sleep disturbances are a major health issue. “[They] are frequent in aging, as well as in neurological and psychiatric disorders,” he continued. “It is important to test if our findings on varying noradrenaline levels can explain hyperarousal that characterizes conditions such as anxiety disorders and PTSD.

“If so, [we need] to build on these findings to develop novel methods to improve sleep quality,” he concluded.

The findings were published in the journal Science Advances.

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