Learning doesn’t just happen during your waking hours . The time you spend sleeping is essential for your ability to remember and organize what you’ve learned, improve newly acquired skills, and forge new insights, as well as determine what to remember and what to forget.
Understanding the precise nature of how sleep shapes our memories is complex and ongoing. Nevertheless, the research to date underscores that the hours we spend asleep are crucial for the dynamic process of learning and memory.
Why we need to sleep to remember information
Have you ever wondered why we spend about two-thirds of our lives awake and one-third asleep? Robert Stickgold, professor of psychiatry at Harvard Medical School and co-author of When Brains Dream, has an explanation: “For every two hours you spend taking in new information during the day, the brain needs to go offline and sit with those external inputs for an hour, to go back over it and figure out what to do with it.” And that, he says, can only happen during sleep.
While we’re awake and constantly receiving new input, we can’t also be processing that information, Stickgold explains. (If you’ve ever zoned out while talking to someone, you’ve experienced this firsthand: starting to think about something else temporarily shuts down the ability to take in new information.)
After a full day of taking in new information and stimuli, we retreat into sleep to sift through it all. In fact, the brain’s need for downtime is a likely reason why sleep evolved in the first place, he says: Sleep provides time for the brain to “turn off all that input and concentrate on processing what it recently learned.”
“Encoding information is easy,” Stickgold points out. “Interpreting it, deciding what to do with it, deciding what it means is hard, so we need that time to sort that out.”
Encoding
The encoding phase, when new information is first being acquired, occurs while we’re awake. This can include what Stickgold termed “quick and dirty memories,” such as looking up a phone number and remembering it long enough to dial it. “I think it’s fair to assume that of the initial memories we encode, we probably lose 95 to 99% of them,” he says.
There’s also an initial consolidation that takes place while we’re awake, although it’s not enough to form lasting, long-term memories. For that, Stickgold says, you need a full night’s sleep.
How memories are stored during sleep
Initial, or short-term, memories are generally described as first being stored in the brain’s hippocampus, and then moved into long-term storage in the cortex (the gray matter that forms the brain’s outer layer) during sleep.
More specifically, researchers think that the new information is initially encoded in both the hippocampus and the cortex, which work together to select and preserve it based on factors such as relevance and emotional intensity this is known as the active systems consolidation hypothesis. An initial memory will include multiple aspects that are parceled out to different parts of the cortex, Stickgold explains. When recalling a conversation, for example, the memory of what the other person was wearing is stored in the visual cortex area, while the words that were spoken are stored in the speech centers as well as the auditory centers of the cortex. “You remember the sound of their voice saying it, but you also remember the words,” Stickgold says.
Initially, the hippocampus keeps the master list of where these various “memory traces” are stored. Then, during sleep, it reactivates all of these various aspects, “so the whole constellation of the memory in the cortex is re-activated,” says Stickgold. As the memory is re-activated, the connections between these memory traces are strengthened, forming a more cohesive memory within the cortex that no longer requires the hippocampus to jump-start the process.
Not only does this process strengthen and preserve memories, it clears out the hippocampus so it’s prepared to take in new information and begin the process anew the following day.
While analogies such as thinking of memories as being moved from one site to another for long-term storage are helpful, Stickgold cautions that these are generalized descriptions of the actual processes taking place. “You can’t go into the brain and find a memory,” Stickgold points out. “At a cellular and molecular level, we still can’t walk through how a memory is formed, how it’s stabilized, how it’s recalled, and how it’s brought back into memory. So we talk at this higher, almost metaphorical level. And we’re really just describing the behavioral outcomes we see.”
“We say the memories are stabilized. But what we’re really saying is that after you sleep, if we teach you new information that should compete with it, you don’t forget [the previous information].”
Is memory associated with a specific sleep stage?
The various sleep stages most likely work together in a complementary manner, as research to date by Stickgold and many others has shown.
In fact, the interplay of the various sleep stages may even be why they evolved in the first place, Stickgold says, with each stage “optimized for different types of memory processes.”
There is no one stage of memory that’s most important for sleep. Other than Stage 1 (the transition to sleep), all of the stages of sleep play essential roles in retaining and processing information.
Stage 2
Also known as light sleep, stage 2 is important for learning and perfecting new motor skills, such as buttoning a shirt or playing the piano. More specifically, research shows that motor skills enhancement corresponds to the frequency and strength of the sleep spindles that occur predominantly during this stage.
These spindles, which resemble the spindles on a spinning wheel when mapped on an EEG, have also been shown to reactivate memories, which helps further strengthen them.
Stage 3
Stage 3 (slow-wave sleep) is essential for declarative learning, or learning new information. It’s also the stage most closely associated with transferring short-term memories to long-term storage. The slow waves that are the hallmark of Stage 3 drive the re-activation of the various aspects of a memory, with the hippocampus pointing the way to the various memory traces stored in different parts of the cortex so they can be consolidated and strengthened.
This sequence, with the slow waves of stage 3 occurring after the sleep spindles of stage 2, is thought to be important for processing multiple memories without having them compete with one another.
REM
REM (rapid eye movement) sleep, which follows these stages, is thought to help further consolidate these memories, although this is still being explored.
During REM sleep, the brain is looking at how new information fits with older, related memories. “REM provides a more nuanced interpretation of what you’ve learned,” Stickgold says.
REM sleep is also important for problem-solving and creativity. During this stage, our brain is able to make new associations between recently acquired information and also combine it with previously stored knowledge in order to forge new insights.
In addition, REM sleep plays a key role in processing and interpreting emotional memories. Stickgold likens the process to sitting down with a therapist to figure out ‘What do these things actually mean?’. As part of this process, REM sleep helps to weaken the emotional impact of intense memories by gradually transforming them. An exception to this occurs in people with post-traumatic stress disorder, who relive the triggering events repeatedly in their dreams rather than slowly processing them.
The effects of poor sleep
Given the essential role of sleep in shaping and storing memories, getting too little sleep can hinder the process.
For example, even though the sleep that takes place after learning receives more focus, being sleep-deprived prior to the fact also plays a role. In one study Stickgold co-authored, participants were kept awake for 36 hours, then asked to memorize lists of words that were considered to be negative, neutral, or positive. They were then allowed two full night’s sleep before being tested on their word recall. Even after this catch-up sleep, however, the researchers found that participants retained far more of the negative words than the neutral or positive ones.
“If you experience something when you’re sleep deprived, it appears that you remember the negative parts of it much more intensely than the neutral and good parts,” Stickgold explains.
Numerous other studies have found that inadequate sleep after learning new information hinders our ability to remember what we’ve learned. In one study of more than 479,000 adults, those who slept just three to six hours did worse on cognitive tests involving working memory than their counterparts who’d gotten seven hours of sleep.
In addition to sleep deprivation, sleep fragmentation or poor-quality sleep can also interfere with the process of memory. Age also plays a role: not only does Stage 3 (slow-wave) sleep decrease from early adulthood on, sleep fragmentation and issues such as insomnia increase.
Our evolving memories
The process of remembering is a dynamic process that’s shaped by these various stages — not just right after new information is taken in, but over a lifetime.
Given that memories aren’t simply consolidated and stored, Stickgold prefers the term “memory evolution to “memory consolidation,” reflecting the many ways that sleep shapes our memories. The aspects of a memory that we remember may change over time, reflecting their ongoing relevance to our lives and their shifting interactions with new memories.
“It’s not done after one night,” Stickgold says. “Your memories continue to evolve over a whole lifetime.”
