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.
Imagine your memory is a warehouse. Each night as you fall asleep the night crew begins its shift and takes stock of the warehouse’s inventory and any available space for new deliveries.
“The brain is very busy when we sleep, repeating what we have learned during the day. Sleep helps reorganize memories and presents them in the most efficient way. Our findings suggest that memories are dynamic, not static. In other words, memories, even old memories, are not final. Sleep constantly updates them,” explains lead study author Maksim Bazhenov, Ph.D., professor of medicine at UC San Diego, in a release. “We predict that during the sleep cycle, both old and new memories are spontaneously replayed, which prevents forgetting and increases recall performance.”
Beyond just maintaining old memories, new memories are “encoded” and consolidated during sleep as well.
“We learn many new things on a daily basis and those memories compete with old memories. To accommodate all memories, we need sleep,” professor Bazhenov says.
But, what exactly does it mean to consolidate and encode new memories? Our memories stretch much further than recollections of big events, birthdays, or parties. Human memory is a key part of overall human intelligence and our capacity to learn. New memory consolidation is what allows our minds to learn and remember one skill on Tuesday, and then learn and recall a different ability on Wednesday.
To better illustrate this point, the study’s authors used the example of learning how to play golf and tennis.
“When you play tennis, you have certain muscle memory. If you then learn how to play golf, you have to learn how to move the same muscles in a different way. Sleep makes sure that learning golf does not erase how to play tennis and makes it possible for different memories to coexist in the brain,” professor Bazhenov notes.
So, sleep also prevents our new memories and learned skills from blurring together and overlapping with older experiences. Memory differentiation is perhaps even more important than just recollection; trying to use a baseball swing to play golf is going to lead to a whole lot of mulligans.
Researchers used a series of computational models to simulate various brain states (sleep, awake). These simulations provided a glimpse into what happens in the mind and memory as an individual sleeps.
Everyone’s memory tends to fade a bit as they grow older, but the team at UCSD says their findings could one day lead to “stimulation techniques” that boost memory processes during sleep.
Crossing over to the world of technology for a moment, professor Bazhenov even suggested that advanced AI and robotics systems may in the future be programmed to enter into a “sleep mode” that helps consolidate newly learned information just like a night of sleep would for a human.
“We may need to add a sleep-like state to computer and robotic systems to prevent forgetting after new learning and to make them able to learn continuously,” he comments.
Regardless of whether or not the robots of the future will be dozing off with the rest of us, these findings certainly provide some food for thought (or food for sleep). The next time you wake up from a dream that was filled with old places and people from your past, now you know why.
The full study can be found here, published in eLife.