Pupils reveal how active the brain is - even during sleep

We don’t give much thought to what happens to our eyes when we sleep. Our eyelids close, and the world around us recedes into darkness. But below those closed lids, a lot more is happening than you might think.

Researchers have found that the size of our pupils changes constantly while we sleep – sometimes expanding, sometimes contracting, and sometimes switching back and forth in a matter of seconds.

Researchers at ETH Zurich have been investigating these changes, and their research undermines a widely accepted assumption regarding sleep.

“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,” said Caroline Lustenberger, one of the researchers involved in the study.

Significance of pupil size

Many assume that sleep is a low-energy state, but this study challenges that notion. The researchers found that even in resting states, our brains fluctuate between different levels of activation.

This is similar to what other studies have found in rodents, which also show slow shifts in activation levels while asleep.

To measure these changes, the scientists developed a new method for observing pupil movements in sleeping individuals.

Since the brain regions that regulate activation levels are deep within the brainstem, they have been difficult to measure directly.

Pupil size is known to reflect activation levels when we’re awake, so it serves as a useful marker for activity in these deep brain areas during different states of consciousness.

Pupil size and brain activity

Measuring pupil movement in sleeping people is tricky – after all, our eyes naturally close when we drift off.

To address this, the researchers devised an unusual approach. They used a special adhesive technique with a transparent plaster to keep the test subjects’ eyes open for several hours while 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,” explained study lead author Manuel Carro Domínguez.

The data collected from these experiments showed a strong connection between pupil size and different sleep stages.

The researchers found that changes in pupil size corresponded with specific patterns of brain activity, such as sleep spindles and deep sleep waves – both of which play key roles in memory consolidation and sleep stability.

Interestingly, they also discovered that the brain’s response to sounds varies in intensity depending on the level of activation reflected in the pupil.

Pupil changes and brain activity

One of the brain regions believed to regulate these changes is the locus coeruleus, a tiny area in the brainstem. In animals, this structure is known to help control sleep stages and wakefulness.

While the researchers could not directly prove that the locus coeruleus is responsible for the pupil changes they observed, they suspect it plays a role.

“We are simply observing pupil changes that are related to the level of brain activation and heart activity,” said Lustenberger.

To investigate this further, the team plans to conduct follow-up studies using medication to influence the activity of the locus coeruleus.

This will help them determine whether it directly controls pupil dynamics during sleep and how these shifts in activation impact sleep quality and function.

Pupils and sleep disorders

Beyond basic research, understanding how pupils change during sleep could have practical applications.

The researchers believe these findings could help in diagnosing and treating sleep disorders and other conditions linked to brain arousal dysfunctions.

The experts are particularly interested in whether pupil changes during sleep might indicate disorders such as insomnia, post-traumatic stress disorder, or even Alzheimer’s disease.

“These are just hypotheses that we want to investigate in the future,” said Lustenberger.

Another potential application is in hospital settings, where this method could be used to monitor wakefulness in coma patients or to improve the accuracy of sleep disorder diagnoses.

By using the pupil as a “window” into brain activity, this approach could open new doors in sleep medicine and neuroscience.

The study highlights an important truth: sleep is far from a passive state. Even as we rest, our brains are busy regulating activity levels – something we can now see reflected in the ever-changing size of our pupils.

The full study was published in the journal Nature Communications.

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