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We often take sleep for granted, assuming our brains simply power down for the night. But new research suggests otherwise, revealing that even in deep slumber, our brains remain remarkably active.
The Mystery of Pupil Movements During Sleep
Researchers at ETH Zurich have discovered that the size of our pupils fluctuates throughout sleep—expanding, contracting, and even switching rapidly between states. This unexpected phenomenon challenges the long-held belief that sleep is a period of low brain activation.
Caroline Lustenberger, a lead researcher in the study, explains: “These dynamics reflect the state of arousal, or the level of brain activation in regions that are responsible for sleep-wake regulation. These observations contradict the previous assumption that, essentially, the level of arousal during sleep is low.”
Significance of Pupil Size and Brain Activity
While many believe sleep is a passive state, this study proves that the brain continuously shifts between activation levels, even in deep sleep. Similar research in rodents has shown comparable fluctuations, suggesting that this is a fundamental feature of mammalian sleep.
To understand these changes, the scientists developed an innovative method to monitor pupil movement in sleeping individuals. Since the brain regions responsible for arousal regulation are located deep within the brainstem, direct observation is challenging. However, because pupil size is known to correlate with arousal levels when we are awake, it serves as a valuable indicator of brain activity during sleep.
Innovative Techniques for Measuring Pupil Activity
Observing pupil size in sleeping individuals posed a challenge. Since our eyes naturally close, the researchers devised a unique approach: they used a special transparent adhesive plaster to keep test subjects’ eyes open as they slept.
“Our main concern was that the test subjects would be unable to sleep with their eyes open. But in a dark room, most people forget that their eyes are still open and they are able to sleep,” said study lead author Manuel Carro Domínguez.
Implications for Sleep Science and Medicine
The data collected revealed a strong correlation between pupil size and sleep stages. Changes in pupil size mirrored specific brain activity patterns, including sleep spindles and deep sleep waves—both crucial for memory consolidation and sleep stability.
Additionally, researchers found that the brain’s response to external sounds varied depending on the level of activation reflected in the pupil size. This suggests that different sleep stages influence how our brains process external stimuli.
One of the brain regions thought to regulate these changes is the locus coeruleus, a tiny area in the brainstem. This structure is already known to play a key role in sleep regulation and wakefulness in animals.
Future Applications and New Insights
The findings from this study may have practical applications in sleep medicine. By analyzing pupil size, doctors could gain deeper insights into sleep disorders and improve diagnoses. Furthermore, this technique may help monitor wakefulness in coma patients, potentially aiding in patient care and rehabilitation.
By using pupil dynamics as a “window” into brain activity, this research sheds new light on the complexity of sleep. Even as we rest, our brains are actively regulating various functions—something now visible through the tiny fluctuations in our pupils.
The full study was published in Nature Communications.
Disclaimer: This article is for informational purposes only and does not constitute medical advice. For concerns regarding sleep health, please consult a medical professional.
