Summary: A new study shows that pupil size continues to change throughout sleep, reflecting ongoing shifts in brain activation. Researchers at ETH Zurich developed a safe method to keep participants’ eyes gently open during overnight recordings, enabling continuous measurement of pupil dynamics without interrupting sleep.
The team found that these pupil fluctuations align with sleep stages, characteristic brain waves, and the brain’s responsiveness to sound. The findings indicate that the sleeping brain remains far more dynamic than previously assumed and suggest new avenues for diagnosing and treating sleep and neurological disorders.
Key facts
- Pupil fluctuations: Pupil size varies continuously during sleep and reflects shifts in brain activation.
- Linked to sleep physiology: Pupil dynamics correlate with sleep stages, sleep spindles, and slow waves tied to memory consolidation and sleep stability.
- Diagnostic potential: Monitoring pupil behavior during sleep may offer a noninvasive marker for sleep disorders and other neurological dysfunctions.
Source: ETH Zurich
Although our eyelids are usually closed during sleep, the brain remains active beneath them. Researchers from the Neural Control of Movement Lab at ETH Zurich—led by Caroline Lustenberger, Sarah Meissner and Nicole Wenderoth—have shown that pupil size does not remain static during sleep. Instead, pupils fluctuate over seconds to minutes, reflecting alternating periods of higher and lower brain activation.
“These dynamics reflect the level of arousal—the moment-to-moment activation of brain systems that regulate sleep and wakefulness,” explains Lustenberger. The discovery challenges the common assumption that arousal is uniformly low during sleep; instead, the brain alternates between more and less active states even when a person appears to be resting.
The ETH Zurich observations in humans align with recent animal studies showing slow, spontaneous fluctuations in arousal. This cross-species consistency strengthens the idea that such fluctuations are a fundamental feature of sleep.
A practical method for a long-standing question
Key brain regions that regulate arousal are located deep in the brainstem, which makes them difficult to study directly in sleeping humans. To overcome this, the researchers used pupil size as an accessible proxy: pupil diameter has long been linked to arousal level during wakefulness, so it can serve as an indirect indicator of deeper brain activity during sleep.
To record pupils overnight, the team developed a gentle adhesive technique: with a transparent medical film they safely kept the right eye open while participants slept in darkness. Contrary to concerns, most volunteers were able to fall asleep with their eye held open because in a dark room the sensation was easily forgotten, says Manuel Carro Domínguez, the study’s lead author who designed the method.
Analysis showed that pupil size changes are tied not only to broad sleep stages but also to specific electrophysiological events, such as clusters of sleep spindles and large-amplitude slow waves. Sleep spindles are associated with sleep resilience and memory processes, while slow-wave activity supports restorative functions. The researchers also observed that the brain’s response to auditory stimuli varied according to the pupil-defined activation level—larger or smaller evoked responses corresponded to different pupil states.
A candidate regulator of these fluctuations is the locus coeruleus, a small but influential nucleus in the brainstem known in animal studies to modulate arousal and transitions between sleep and wake. The current study did not prove direct control by the locus coeruleus; it documents correlative links between pupil size, brain activation, and cardiac activity. Follow-up work will test whether pharmacologically targeting the locus coeruleus alters pupil dynamics and, in turn, sleep physiology.
Clinical and research implications
Understanding pupil dynamics during sleep could have meaningful clinical applications. If pupil fluctuations reliably index arousal-system function, they might serve as an accessible biomarker for conditions that involve arousal dysregulation—such as insomnia, post-traumatic stress disorder, and potentially neurodegenerative diseases like Alzheimer’s. The researchers caution that these ideas remain hypotheses that require further study.
A longer-term goal is to translate the technique beyond sleep laboratories. Portable and user-friendly methods to monitor pupil dynamics could be useful in hospital settings to assess consciousness in coma patients or to improve diagnostic precision for sleep disorders. Viewing the pupil as a noninvasive window into brain state may open new pathways in sleep medicine and cognitive neuroscience.
About this research
Author: Marianne Lucien
Source: ETH Zurich
Contact: Marianne Lucien – ETH Zurich
Image: Image credit: Neuroscience News
Original research (open access): “Pupil size reveals arousal level fluctuations in human sleep” by Sarah Meissner et al., Nature Communications.
Abstract
Pupil size reveals arousal level fluctuations in human sleep
Animal studies have uncovered complex arousal dynamics that support sleep resilience and memory consolidation. Translating these findings to humans has been difficult because measuring arousal fluctuations during natural sleep poses methodological challenges. In this study, researchers recorded pupil size throughout overnight sleep by safely taping the right eye open and examined how pupil state influenced cortical responses to auditory stimulation.
The results show that pupil dynamics shift in relation to key sleep events across multiple time scales. Notably, pupil size was inversely related to the occurrence of sleep spindle clusters—an electrophysiological marker of sleep stability. Pre-stimulus pupil size also shaped the brain’s evoked response, especially in delta-band power, which is associated with restorative sleep functions. Continuous pupillometry during sleep thus reveals how arousal levels interact with sleep oscillations and may provide a practical readout of sleep physiology and resilience.