Just like a nostalgic grandparent flipping through old photo albums, our brains constantly replay memories from past events in our lives as we sleep.

It may seem overly sentimental at first, but our minds aren’t just looking to reminisce and remember the good times. All of that brain activity while dreaming serves to strengthen and preserve existing memories, all while simultaneously finding some room for any new memories we may have made over the previous day.

Those are the main findings from a fascinating new study just released by the University of California, San Diego that investigated neural activity during sleep. The research team at UCSD says that no memory is set in stone within our minds; any memory can be lost, and sleep is when our minds rejuvenate old memories via replay and refine/make room for new memories.