New research points to noradrenaline (or norepinephrine), a neurotransmitter that’s secreted during stressful or dangerous situations, is what mediates our brain’s ability to block sensory responses during sleep.

Although we don’t remember much after the fact, our brains remain very active during sleep. However, the information it receives from our senses is heavily filtered — we don’t consciously perceive it, and only certain stimuli are able to rouse us from our sleep. Noradrenaline seems to underpin the process, according to the findings of a series of studies led by researchers at Tel Aviv University (TAU).

Peaceful slumber

“In these studies, we used different, novel approaches to study the filtering of sensory information during sleep and the brain mechanisms that determine when we awaken in response to external events,” explains Prof. Yuval Nir, who led the research for the three studies.

The first study, published April 1st in the Journal of Neuroscience, used rat models to show that neurons in the auditory cortex have similar responses to stimuli whether the animals were asleep or awake. Neurons in the perirhinal cortex however, which are involved in complex conscious perception and memory associations, showed much weaker responses during sleep.

He says that while our brains don’t create a conscious perception of sounds during sleep, “basic analysis of sound remains” active. Furthermore, “initial and fast responses are preserved in sleep” but more complex ones — which require coordination between different areas of the cortex — are significantly disrupted.

These findings, explains TAU doctoral student Yaniv Sela, challenge the current assumption that the thalamus is responsible for blocking incoming signals to the cerebral cortex while we sleep.

The second study, published a week later in Science Advances, reports that the locus coeruleus mediates the brain’s ability to disengage from sensory information during sleep in rats. The locus coeruleus is a small region of the brainstem and the main producer of noradrenaline in the brain.

“The ability to disconnect from the environment, in a reversible way, is a central feature of sleep,” explains TAU doctoral student Hanna Hayat, lead author of the study. “Our 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.”

The team monitored activity in the locus coeruleus of sleeping rats and exposed them to different sounds to see which would be able to wake them up. Activity in this area of the brain could reliably predict whether the animals would awake in response to a sound, they write.

To check their findings, the team used optogenetics (the use of genetically-modified cells that can be turned on or off through exposure to light) to inactivate the locus in sleeping rats — which dramatically reduced their likelihood of waking up in response to sound. Alternatively, the team reports that increasing noradrenaline activity in the locus coeruleus made the animals wake up more frequently in response to sound.

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

Heightened arousal in this brain area, the team explains, could explain why light sleepers or individuals experiencing stressful times have trouble staying asleep.

The first paper “Sleep Differentially Affects Early and Late Neuronal Responses to Sounds in Auditory and Perirhinal Cortices” has been published in the Journal of Neuroscience.

The second paper “Locus coeruleus norepinephrine activity mediates sensory-evoked awakenings from sleep” has been published in the journal Science Advances.