Not only must your brain maintain a vast archive of existing memories, but it also has to keep track of new stuff each day. That requires getting ready to deal with new memories before they occur, even while processing old ones.
It's well-known that sleep is important for memory and learning, among other things, but we're still figuring out many of the precise mechanisms involved.
Traditionally, it's seen as a backwards-facing process: We experience something, the memory of it is then processed and preserved during subsequent nights of sleep, and we can recall it at will later.
According to a new study, however, sleep seems to help us process not just past memories, but also future ones. In addition to consolidating and conserving memories while we sleep, the study suggests our brains are also actively preparing us to record upcoming events.
Memory is a broad, multifaceted phenomenon that helps make us who we are, and from our subjective perspective it can seem almost magical, somehow beyond discrete components.
It's not, though. At a cellular level, groups of specialized neurons known as engram cells physically encode our life experiences in a format we can recall later.
Previous research has made it clear sleep is crucial for this process to work properly, but many physiological details remain unclear. On top of merely storing memories, our brains also perform incredible feats of processing and organization, much of which occurs during mental downtime.
For the new study, researchers in Japan sought to learn more about the role sleep plays in memory processing, including the preparation for memorable experiences that haven't happened yet.
The authors used an imaging system for freely moving mice that can reveal both engram and non-engram cells across the stages of memory processing. They were able to track neuronal activity before, during, and after memorable events the mice experienced.
This shed new light on the behavior of specific neuron populations under various cognitive conditions, including while mice slept both before and after learning experiences.
The results highlighted two parallel processes unfolding in post-learning slumber. First, engram cells that initially encoded a memory showed predictable reactivation patterns, part of the brain's well-documented procedure for consolidating memories during sleep.
The study also revealed another interesting population of neurons, which weren't yet associated with specific memories. These "engram-to-be cells," as the researchers call them, grew increasingly synchronized as mice slept after learning. Later, the same neuron groups went on to encode new, different memories.
"Engram-to-be cells exhibited increased coactivity with existing engram cells during sleep, suggesting that this interaction helps shape new memory networks," says co-author Kaoru Inokuchi, a professor of biochemistry at the University of Toyama in Japan.
The researchers also developed a neural network model to simulate activity in the hippocampus, hoping to further illuminate the underlying mechanics.
The model pointed to synaptic depression and scaling, phenomena known to modify neuronal connections during sleep, as likely important for organizing these engram-to-be cells – a service that dwindled in the model when those mechanisms were disabled.
Engram and engram-to-be cells showed intriguing co-activation in post-learning sleep, the study found, hinting at some coordination or even data transfer between the neural networks for past and upcoming memories.
This suggests sleep quality between learning events may influence not only our retention of what we've learned so far, but also how well we'll retain new information in the near future.
While more research is needed, this could be valuable insight for education and treatment of memory disorders, and it could help uncover novel ways to help people maximize their cognitive performance.
"We believe that manipulating brain activity during sleep or sleep patterns may uncover methods to enhance memory by unlocking the brain's latent potential," Inokuchi says.
Above all, however, these findings add to the mountain of existing evidence that we should all take sleep seriously.
"We want people to understand that sleep is not just about rest – it plays a crucial role in how the brain processes information," Inokuchi says. "With that in mind, we hope everyone will begin to value sleep more and use it as a way to improve their overall quality of life."
The study was published in Nature Communications.