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ETH Zurich researchers uncover surprising insights into sleep dynamics
A groundbreaking study has challenged long-held assumptions about brain activity during sleep. For the first time, researchers have observed how pupils react over several hours of sleep, revealing constant fluctuations in size. The study, published in Nature Communications, indicates that the brain is far more active during sleep than previously assumed.
A Window to the Sleeping Brain
Although eyes remain closed during sleep, a team led by Caroline Lustenberger, Sarah Meissner, and Nicole Wenderoth from the Neural Control of Movement Lab at ETH Zurich discovered that pupil size continuously changes—sometimes within seconds, other times over several minutes.
“These dynamics reflect the state of arousal, or the level of brain activation in regions responsible for sleep-wake regulation,” explains Lustenberger. “These observations contradict the previous assumption that arousal levels during sleep remain low.”
Rather than a uniform state, the brain appears to shift between high and low activation levels, a phenomenon previously observed in rodent studies. The findings suggest that even during deep sleep, the brain undergoes significant fluctuations.
Innovative Methodology
Measuring brain activity in deep-seated brainstem regions has historically posed challenges. Traditional techniques are complex and have not been widely applied to this context. To overcome this, the ETH researchers turned to pupil measurements, as pupil size is a known marker of brain activation during wakefulness.
The team developed an innovative approach using a special adhesive technique and transparent plaster to keep participants’ eyes open during sleep. “Our main concern was that subjects wouldn’t be able to sleep with their eyes open,” explains lead author Manuel Carro Domínguez. “However, in a dark room, most people forget their eyes are open and are still able to sleep.”
Analysis of the data linked pupil fluctuations not only to sleep stages but also to specific brain activity patterns, including sleep spindles and deep sleep waves, which are essential for memory consolidation and sleep stability. Additionally, the brain’s reaction to sound stimuli varied with the level of activation, as reflected in pupil size changes.
Future Research and Medical Implications
A central regulator of brain activation is the locus coeruleus, a small brainstem region involved in sleep and waking regulation. While the researchers couldn’t confirm its direct role in pupil changes, future studies aim to manipulate its activity with medication to investigate its potential influence.
Understanding pupil dynamics during sleep may have important implications for diagnosing and treating sleep disorders, such as insomnia and PTSD, as well as neurodegenerative conditions like Alzheimer’s. The research team also hopes to extend their methodology beyond sleep labs to clinical settings, where it could assist in monitoring coma patients and diagnosing sleep disorders more accurately.
“Our findings open up exciting possibilities,” says Lustenberger. “But there is still much to explore.”
Disclaimer
This article is based on the findings of a scientific study and is intended for informational purposes only. Further research is required to confirm potential medical applications. Readers should consult healthcare professionals for concerns related to sleep disorders or neurological conditions.
