Marc N. Coutanche Ph.D. :
One of sleep’s key functions is to stabilize and integrate new memories into our brain’s long-term memory systems, a process known as “consolidation.” This idea is now widely accepted, though it wasn’t always. The details remain very much under investigation, however: what kinds of memories are prioritized, how consolidation unfolds, and exactly when different forms of consolidation occur. In this post, I’ll be focusing on the “what” of the many things that we learn each day, which kinds of information does sleep strengthen?
One important factor is how well the material was learned in the first place. In studies where participants learn pairs of words (e.g., apple-chair) to varying degrees, it has been the weakly learned associations (say, recalled about 60% of the time) that benefited most from sleep (Drosopoulos et al., 2007). In contrast, strongly learned pairs show less improvement. This makes intuitive sense: with limited time and processing capacity, our brain prioritizes reinforcing memories that are still fragile, rather than spending some of that precious sleep computational power on memories that are already well established.
Sleep also plays a role in connecting the dots between pieces of information. In one of our studies, we had participants learn spatial relationships between fictional locations, such as learning that a grocery store is west of a coffee shop, and that the coffee shop’s west of a library (Coutanche et al., 2013). When asked to construct a map of the area from memory, those who had slept after learning performed significantly better. Investigating this further revealed that participants in the sleep group weren’t just better at recalling the relationships we had taught them but were better able to make inferences about untaught ones. For instance, even if we hadn’t explicitly said that the grocery store was west of the library, their maps reflected this information. As the participants slept, their brains were forming new connections between items, allowing them to later recognize untaught relationships. Many of us have had the experience of an ‘aha’ moment one morning, where a solution seems to come to us after sleeping on a problem. This may be analogous to our findings – both situations involve recognizing new relationships and ‘connecting the dots’ in new ways.
Emotionally charged memories also receive special treatment during sleep. When people view a series of images, those with emotional content are more likely to be later remembered, particularly if there was a period of sleep between learning and recall (Wagner et al., 2001). Similarly, if information is associated with a reward during learning, it is more likely to be consolidated, as if the reward tags the material as being important and worthy of future consolidation (Sterpenich et al., 2021).
While most of this post has focused on declarative memories (facts and events), sleep also benefits non-declarative memory, like the motor sequences (Kuriyama et al., 2004) involved in playing a piano concerto or performing a basketball layup and the statistics of our environment (Durrant et al., 2011), especially if they were difficult to learn.
When thinking about ‘what’ information is consolidated by sleep, we should also consider how the information is changed by the process. In fact, in some cases, the effects of sleep aren’t visible in straightforward memory tests but become clear under challenging conditions. In one classic paradigm, participants first learn word pairs (A-B), then sleep or stay awake, and are later asked to learn new pairs involving the same words (A-C). Participants are then asked to recall the initial A-B pairs. Those who didn’t sleep tend to have their earlier A-B memories disrupted by the newer A-C ones. In contrast, participants who slept after they learned the initial pairs are more resistant to the interference (Ellenbogen et al., 2006). In these cases, sleep doesn’t make memories more generally accessible, but makes them more stable and resilient in the face of competing information.
The author is Psychologist and Research Scientist.
In sum, sleep selectively strengthens weak associations, helps us infer new relationships between bits of knowledge, prioritizes emotional and rewarded information, and protects memories from interference. It’s not just a passive resting state but is an active process that shapes what we remember and how we use it. This is a very active and fast-moving research field so it will be exciting to see what we will learn next about sleep’s role in forming memories.
