When you’re sleeping at night, your body is resting, but your brain is still hard at work. It goes through everything you did that day, saving some things as memories and letting others go. Scientists have always known that sleep helps us remember things, but now they’ve discovered a new way the brain protects old memories while adding new ones.
Researchers at Cornell University found something surprising: the pupil (the black part of your eye), which usually responds to light and emotions, also plays a role during sleep. By watching small changes in the pupils of sleeping mice, they saw a special pattern happening in the brain.
What Happens in the Brain During Sleep
The scientists trained mice to do simple things like look for water or treats in a maze. After the training, the mice wore small brain sensors and tiny cameras that recorded their eye movements while they slept.
They focused on non-REM sleep—a sleep stage that’s important for building memories. What they saw amazed them.
Even though the mice’s eyes were closed, their pupils were still moving. The pupils got smaller and larger in quick moments, each lasting about 100 milliseconds (a tenth of a second). These tiny changes matched brain activity.
- When the pupils shrank, the brain replayed new things the mice had just learned.
- When the pupils widened, the brain replayed older memories.
This back-and-forth helped the brain keep new memories and old memories from clashing. It’s like the brain was switching gears—storing new info, then checking on the old stuff. This might be how we avoid forgetting important things when we learn something new.
A Hidden Pattern in Sleep
Before this, scientists thought non-REM sleep was one long memory-building phase. But this study showed there’s actually a hidden structure inside it—a rhythm the brain follows.
Professor Azahara Oliva, who worked on the study, explained:
“These moments are super short—just 100 milliseconds—and we didn’t know they existed before.”
It’s like the brain says: “Time to store new memories… now switch to old memories… back to new,” all night long. This rhythm helps the brain organize everything without mixing things up.
Testing the Theory
To see if this pattern really mattered, the scientists woke the mice up at different times during sleep.
- If the mice were woken during the small pupil phase, they forgot new tasks.
- If they were woken during the large pupil phase, they forgot older tasks.
So, when the brain is interrupted at the wrong time, memory doesn’t work as well. It’s not just about how long you sleep—it’s also about when things happen during sleep.
Why This Matters
Even though this research was done in mice, it could help people too. If human brains work the same way, we could use eye movement to help with learning, memory loss, or even improve artificial intelligence (AI).
Today’s AI systems often forget old information when learning new things. Studying how our brains avoid that might help computers get smarter.
Professor Antonio Fernandez-Ruiz, who helped lead the study, said:
“We think the brain uses a special timing system to keep new learning and old knowledge separate.”
That timing system seems to show up in the pupils—and it could be the missing piece in understanding how memory works.
What’s Next?
Scientists may try this with people to see if our pupils do the same thing during sleep. If they do, we might one day use simple eye-tracking tools to help people learn faster, remember more, or even treat memory problems.
But even without high-tech tools, one thing is clear:
Good sleep is more important than we think. Your brain is doing careful, timed work to protect your past and help you learn for the future.