Though sleep is a time to power down and replenish our energy, our brains remain surprisingly active when we’re at rest. In fact, more goes on behind our closed eyelids than we previously thought, according to a team of researchers that recently analyzed eye movements during slumber.
In a study published Wednesday in the journal Nature Communications, researchers in Switzerland analyzed the pupils of sleeping participants. The study reveals that our pupils constantly change size while we’re unconscious, indicating different levels of brain activation—the brain’s response to stimuli. With further research, doctors might one day use pupil activity to diagnose conditions such as sleep disorders.
Pupil dynamics “reflect the state of arousal, or the level of brain activation in regions that are responsible for sleep-wake regulation,” said Caroline Lustenberger, a neuroscientist at ETH Zürich and co-author of the study, in a university statement. “These observations contradict the previous assumption that, essentially, the level of arousal during sleep is low.”
The locus coeruleus—a part of the brain that regulates activation levels—is located in the brainstem and is notoriously difficult to study while individuals are asleep, according to the researchers. Since the pupil size is known to reflect brain activity, Lustenberger and her colleagues assumed they could also be used to track brain activation during sleep.
The team claims to be the first to observe eye movements during sleep for several hours at a time—and I know what you’re thinking: how? The featured image in this article is not a stock photo—it shows how they actually did it. The researchers simply taped one eye of each participant open, moistened it with eye ointment, and then sealed it behind a transparent bandage.
“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 Manuel Carro Domínguez, a biomedical engineer at ETH Zürich and lead author of the study, who invented the technique.
Overall, the participants’ pupil movement demonstrated that activation levels are constantly changing during sleep. According to the researchers, the finding confirms a biological feature in humans that previous studies had documented in rodents.
The team also noted a link between pupil dynamics during sleep and specific brain activity patterns, including brain waves involved with sleep stability and memory consolidation. Furthermore, they discovered that the intensity of the brain’s reaction to sound depends on the level of its activation, as indicated by the participants’ pupils. However, the study did not prove whether the locus coeruleus directly influences pupil dynamics.
“We are simply observing pupil changes that are related to the level of brain activation and heart activity,” Lustenberger explained. The team aims to investigate this potential dynamic, as well as how activation levels influence sleep, in a follow-up study. If they do find a strong causal relationship, pupil movement might one day be used to detect disorders such as insomnia and post-traumatic stress disorder, or even a comatose patient’s recovery.
We haven’t yet figured out if eyes are windows to the soul—but as the team’s research shows, they’re certainly windows to the brain!
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