Summary: A new study reveals that pupil sizes constantly fluctuate during sleep, reflecting changes in brain activation levels. Researchers have developed new methods to keep subjects’ eyes open, allowing them to monitor these dynamics without disrupting sleep.
They found that pupil changes correspond to sleep stages, brain waves, and reactivity to sound, suggesting that the brain remains very active even at rest. This finding could lead to new ways to diagnose and treat sleep disorders and other neurological conditions.
Important facts
Pupil variation: The size of the pupil changes during sleep, reflecting the level of brain activation. Linked to the sleep stage: pupil dynamics correlate with deep sleep waves and memory-related brain activity.
Source: ETH Zurich
Our eyes are usually closed when we sleep. However, there is a surge in activity occurring under our closed eyelids. A team of researchers led by Caroline Rustenberger, Sarah Meisner and Nicole Wenders of Neurocontrol at the ETH Zurich Athletic Lab, observed that pupil sizes fluctuate constantly during sleep.
It can grow in size and sometimes decrease. These changes can occur within minutes or over a few minutes.
“These dynamics reflect the level of brain activation in the wakefulness or areas that cause sleep-wake regulation,” says Lustenberger.
“These observations contradict previous assumptions that inherently low levels of wakefulness during sleep.”
Instead, these variations in pupil size indicate that the brain is constantly switching between higher and lower levels of activation, even during sleep.
These new findings also confirm humans what other research groups have recently discovered in their rodent studies. Rodents have also been shown to fluctuate slowly at activation levels (known as arousal).
The study by ETH researchers was recently published in the journal Nature Communications.
New ways for old mysteries
The brain regions that control activation levels are located deep within the brain stem, making it difficult to directly measure these processes in humans during sleep. Existing methods are technically demanding and have not yet been established in this context.
Therefore, ETH researchers’ research relies on student measurements. Students are known to exhibit activation levels when a person is awake. Therefore, they can be used as markers of activity in areas that are deeply located in the brain.
ETH researchers have developed new methods to examine changes in people’s students while they are asleep. Special adhesion techniques and clear plaster allowed subjects to remain open for several hours.
“Our main concern is that subjects cannot sleep with their eyes open, but in dark rooms, most people forget that their eyes are still open and they can sleep,” explains Manuel Caro Dominguez, the lead author of the study that developed the technique.
Analysis of the data showed that pupil dynamics are related not only to different stages of sleep, but also to specific patterns of brain activity such as sleep spindles and deep sleep waves.
Researchers also found that the brain responds to sounds at varying degrees of intensity depending on the level of activation reflected in student size.
The central regulator of activation levels is a small area of the brainstem, known as the locus dura. In animals, scientists were able to demonstrate that this is important for regulating sleep stages and awakening.
ETH researchers were unable to demonstrate in this study whether the locus cognate is in fact directly involved in pupil changes.
“We simply observe pupil changes related to brain activation and levels of cardiac activity,” explains Lustenberger.
Follow-up studies allow researchers to use drugs to influence locus charcoal activity, allowing them to investigate how this affects pupil dynamics. They want to discover whether this area of the brain is actually responsible for controlling the pupils during sleep and how changes in levels of activation affect sleep and its function.
Diagnose illness using pupil dynamics
Understanding student dynamics during sleep may also provide important insights for the diagnosis and treatment of sleep disorders and other illnesses.
Therefore, researchers want to investigate whether changes in the pupil during sleep can provide signs of dysfunction in the arousal system. These include disorders such as insomnia, post-traumatic stress disorder, and perhaps Alzheimer’s disease.
“These are just hypotheses we want to investigate in the future,” says Lustenberger.
Another goal is to allow technology to be used outside the sleep room. For example, hospitals that can help monitor awake in coma patients and more accurately diagnose sleep disorders. Therefore, pupils like the windows of the brain can pave the way for new opportunities in sleep medicine and neuroscience.
Research news about this sleep and brain activity
Author: Marianne Lucian
Source: ETH Zurich
Contact: Marianne Lucien – Eth Zurich
Image: Image credited to Neuroscience News
Original research: Open access.
“Pupil size reveals variation in human sleep awakening levels,” says Sarah Meissner et al. Natural Communication
Abstract
Pupil size reveals variation in human sleep awakening levels
Recent animal studies have revealed complex dynamics of arousal levels that are important to maintain proper sleep resilience and memory integration.
In humans, changes in arousal levels are considered to be a critical feature of healthy and pathological sleep, but tracking fluctuations in arousal levels is methodologically challenging.
Here, we measured pupil size, an established indicator of arousal level, by safely taping the right eye during overnight sleep and testing whether pupil size affects cortical response to auditory stimuli.
We show that pupil size dynamics vary as a function of important sleep events across different time scales. In particular, our results show that student size is inversely proportional to the development of sleep spindle masses, a marker of sleep resilience.
Furthermore, pupil sizes that affect elicited responses were found prior to auditory stimulation, particularly at delta power, a marker of some resilient and regenerative functions of sleep.
Recording student size dynamics provides insight into the interaction of wakefulness levels and sleep vibrations.
