Eyes play a critical role during sleep, study finds

In the quiet hours of the night, while your body rests, your brain is busy. It's sorting through the day’s moments, filing some away as lasting memories, and brushing off others. Scientists have long known sleep helps memory, but new research has revealed a hidden process that explains how the brain keeps old memories safe while adding new ones.

Researchers from a Cornell University have discovered that the pupil of the eye—long known to respond to light and emotion—also plays a critical role during sleep. By closely studying tiny changes in the pupils of sleeping mice, they uncovered a delicate dance between memory systems in the brain. This discovery could one day improve how we learn and remember.

To understand what happens in the brain during sleep, scientists studied mice trained to perform simple tasks like searching for water or cookies in a maze. These tasks taught the mice something new every day over the course of a month. After training, the mice were fitted with two special tools: brain electrodes to track neural activity and tiny eye-mounted cameras to watch their pupils as they slept.

The goal was to learn what happens in the brain during non-REM (non-rapid eye movement) sleep, a stage known for memory consolidation. What the researchers found was surprising.

During this stage, the size of the mice’s pupils changed in subtle ways—sometimes shrinking and sometimes widening. These shifts, lasting only about 100 milliseconds, lined up with very specific brain activity.

During moments when the pupils shrank, the mice’s brains replayed memories from the day—the new tasks they had just learned. These short bursts of memory replay, known as sharp wave-ripples, helped store new knowledge in a lasting way. But when the pupils dilated, the brain switched gears and began replaying older, already-stored memories.

This alternating pattern prevented interference between new and old information. The researchers believe this is how the brain avoids “catastrophic forgetting,” a problem where learning something new causes the brain to overwrite older memories.

Until now, scientists thought non-REM sleep was a uniform block of time in which memories were broadly consolidated. But this study revealed a hidden micro-structure within that sleep phase. Assistant Professor Azahara Oliva explained, “Non-REM sleep is when the actual memory consolidation happens, and these moments are very, very short periods of time undetectable by humans, like 100 milliseconds.”

The study showed that these tiny shifts in pupil size mark a rhythm that allows the brain to alternate between storing new information and preserving older knowledge. "It's like new learning, old knowledge, new learning, old knowledge, and that is fluctuating slowly throughout the sleep," Oliva said.

This pattern gives the brain time to manage both kinds of memories without letting them interfere with each other. That back-and-forth could be the key to how humans and animals remember so much, so well, without getting confused or overwhelmed.

To make sure their findings were solid, the team interrupted the mice’s sleep at different points. After waking, the mice were tested on how well they remembered the tasks they had learned.

The results lined up with the theory: when sleep was interrupted during the contracted pupil stage, the mice had trouble remembering the new tasks. But if they were disturbed during the dilated pupil stage, their recall of older tasks suffered instead.

This showed that the timing of memory consolidation really does matter. It’s not just about getting enough sleep—it’s about the brain’s ability to manage its internal schedule while sleeping.

While this research was done in mice, its effects could be wide-reaching. The idea that pupil dynamics during sleep help control memory might lead to new ways to help people learn better or recover lost memories. It could also help scientists design smarter artificial intelligence systems.

Current neural networks used in AI often suffer from “catastrophic forgetting,” much like the brain would without this sleep structure. Learning how the brain avoids that problem could help computer scientists design systems that learn without erasing old data.

Assistant Professor Antonio Fernandez-Ruiz, who led the study with Oliva, hopes the findings will open new doors. “We are proposing that the brain has this intermediate timescale that separates the new learning from the old knowledge,” he explained.

This intermediate timescale, marked by changes in the eye’s pupil, may be the missing piece in understanding how memory systems stay stable while constantly updating.

The next step could involve testing similar processes in humans. If the same pupil-linked stages exist in human sleep, scientists might one day use simple eye-tracking technology to boost memory or improve learning. Treatments for memory loss or cognitive decline might also benefit from this research.

Even without advanced tools, the takeaway is clear: getting quality sleep may be more important than we think. The brain isn’t just resting during sleep—it’s doing careful, timed work to protect your past and prepare for your future.

Note: The article above provided above by The Brighter Side of News.

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