Welcome to the memory storage department – graveyard shift!
This is Sandra Tsing Loh with the Loh Down on Science.
Your sleep is divided into different stages, each helping store memories from the day. But how does your brain sort through WHICH memories to keep?
Enter Hongyu Chang and team at Cornell University. They tracked tiny pupil changes in sleeping mice to measure changes in brain activity.
And? The sleeping brain switches between two modes of pupil size! One mode with large, dilated pupils signaled when their brains were organizing and replaying older memories.
Meanwhile, smaller, constricted pupils signaled their brain saving NEW memories! Disrupting this small pupil stage with pulsating light even caused memory recall problems!
So make tonight memorable; dim the lights and leave it to the night crew!
Reference: Chang, H., Tang, W., Wulf, A.M. et al. Sleep microstructure organizes memory replay. Nature, (2025). https://doi.org/10.1038/s41586-024-08340-w