The Sleeping Brain Is a Marvelous Memory Making Machine

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For as long as I can remember, I’ve wanted to learn Japanese.

Linguists often say that the best way to learn a foreign language is to immerse yourself in it, and that’s what I did. For a while, I put post-it notes on items with their Japanese names on them, watched anime with the subtitles off and even fell asleep to kawaii Japanese podcasts whispering in my ear.

Learn by osmosis, right? Maybe even as I sleep something will stick, I thought.

scientists-find-sleep-ultimate-memory-aid-3 It sounds utterly nonsensical — learn while you’re unconscious?! — but the idea of sleep learning has thrived for centuries. Eerily described in Brave New World by Aldous Huxley in the 1930s, sleep was once thought to be a subliminal state similar to hypnosis, in which the person is susceptible to messages from the outside world.

Perhaps because of this dystopian sci-fi association, early neuroscientists lambasted sleep-learning — something desirable, marketable and potentially abusive — as quackery.

As it turns out, they might be wrong.

Sleep may be the brave new world for learning. A recent flood of high profile studies suggest that our nightly slumber profoundly boosts what we’ve learned in the previous day, making the information stick.

But here's the crazy cool part: animals and humans may have the ability to absorb new information during sleep, without ever being consciously aware of the learning process.

Let me be clear: it’s an extremely contentious idea, one far from settled. But here’s what we know. The story starts with a seahorse-shaped brain structure called the hippocampus and the electrical waves that oscillate through it after the lights go out.

Record, Replay, Retain

As you go about your day, various experiences are transformed into electrical impulses that spread through neural networks in the hippocampus. These waves of activity gently tweak the connections between neurons called synapses, making some stronger and others weaker.

Roughly speaking, memories are temporarily stored in these changes in synaptic strength, much like information in a computer is stored by swapping 0s and 1s.

Newly formed memory traces are extremely fragile. Without further processing, they fade into the brain’s background chatter. The hippocampus acts as a sorting facility for these newcomers — only the important ones are consolidated into existing memory networks and transferred to the cortex for more permanent safekeeping.

But what determines which memories are important enough to require further processing? This is where sleep comes in.

scientists-find-sleep-ultimate-memory-aid-4In the mid 1990s, scientists discovered a peculiar type of brain activity in deeply asleep rodents while studying spatial learning.

The previous day, the rodents had learned to navigate a new maze to get to a food reward. As they learned, a special type of neuron in the hippocampus called a “place cell” activated in sequence to encode the memory.

Place cells are incredibly specific for a particular location, acting almost like GPS coordinates. By looking at the pattern of place cell activation, scientists can roughly trace the animal’s trajectory through the maze.

The surprise came when the rodents drifted into a deep, dreamless stage of sleep called slow-wave sleep (SWS). During SWS, the brain’s electrical patterns lazily cycle between high peaks and low troughs. Yet periodically, the researchers observed sharp bursts of synchronous electrical activity puncturing these smooth oscillations. They came in incredibly fast frequencies, true to their name “sleep spindles.”

The scientists found these sleep spindles represented highly compressed information — when “stretched out” they were shockingly similar to the place cell activation patterns recorded the previous day. The hippocampus seems to be rebooting previously activated synapses, rehearsing the winning maze trajectory at fast-forward speed.

When scientists disrupted these sharp ripples, the memory faded.

Later studies found that humans also actively replay new information during sleep. What’s more, a study published this week showed that SWS doesn’t just boost signal — it simultaneously inhibits neurons required for forgetting.

Sleep seems to be the all-natural aid for better memory. And by studying how sleep works its magic, scientists have handed us the key to further boosting its effects.

Back in 2007, for example, two teams of German neuroscientists ambitiously demonstrated that a whiff of a familiar scent reactivates memories from the day before. The scientists had a group of medical students remember the location of card pairs on a computer screen. Whenever the students succeeded, the scientists puffed a burst of rose scent into their noses.

About a half hour later, the students took a nap while wearing EEG caps to monitor their brain activity. During SWS, some received the same rose odor — and upon awakening, they performed 15% better on the same card task than the control group that didn’t get the memory cue.

scientists-find-sleep-ultimate-memory-aid-6An even more tantalizing study came in 2012. Neuroscientists at Northwestern University taught musical novices to play melodies on a keyboard and subsequently put them to sleep. Half of the participants had the melody playing on repeat while they slept, and they made far fewer mistakes when asked to replay the melody after waking up.

Auditory cues also worked with language learning: softly playing recordings of Dutch to a group of German speakers as they slept helped them remember the meanings of newly learned Dutch words better.

Learning by osmosis might not be such a crazy idea after all.

Scientists are already trying to boost sleep-learning with brain stimulation. The University of Tubingen’s Jan Born, for example, is using transcranial direct current stimulation (tDCS) to amplify sleep ripples in human participants. He’s also experimenting with sound frequencies that gently deepen SWS.

It’s a natural way of getting the system into rhythm and making SWS more intense, Born said in an interview with BBC.

Sleep Inception

Meanwhile, other scientists are pushing the limits of sleep-learning. Instead of enhancing previously learned information, they’re testing ways to incept new memories into the slumbering brain.

In 2015, a team from Paris published a mind-boggling proof-of-concept.

Scientists recorded activity from place cells of a mouse as it explored a circular chamber. They then picked a single place cell, which encodes a precise location in the chamber. When the cell reactivated during sleep, the scientists zapped the pleasure center of the brain, thus creating a false association between the place cell and an intense sensation of pleasure.

scientists-find-sleep-ultimate-memory-aid-5When the mouse awoke, it dashed towards that location in hopes of getting another pleasant high.

Rather than enhancing previous learning, scientists had formed a new memory during sleep in mice.

Not long after came the first demonstration of memory inception during sleep in humans. A team in Israel wondered whether linking something pleasant — the smell of cigarettes for smokers — to something wholly miserable — the smell of rotten eggs — could help smokers kick the habit.

Both odors were puffed into the volunteer’s nostrils during SWS. Upon awaking, participants smoked 30% less on average the following week. It may not seem like a huge effect, but remember: the participants didn’t have to suffer through the experience. Without ever being consciously aware of the aversive association, they subconsciously reduced their cigarette craving.

Although we don’t know how long the effects last, the therapy is easy and harmless enough for multiple treatments until the smokers kick the habit for good.

A Good Bargain?

We’re just scratching the surface of sleep-learning. The benefits could be enormous. Students may be able to learn faster with minimal extra effort. Addicts may be able to gently disentangle from their drug-of-choice. PTSD sufferers may be able to encode pleasant memories to suppress traumatizing ones that haunt their dreams.

And for anyone with an overly hectic schedule, sleep may provide those bonus hours to master a new career skill, learn a new language or practice playing a new instrument.

But the question is: would you want to?

Sleep-learning may come at a price. So far, no one has studied whether amplifying memories or learning new things during sleep inhibits its other functions.

A recent study showed that during sleep, the brain’s sewage system kicks into high gear, washing out potentially harmful metabolic waste and reducing the chance of developing neurodegenerative diseases like Alzheimer’s. Spring cleaning in the brain needs a lot of energy, some of which would be siphoned away by sleep-learning.

What if the extra edge of better memory now comes at a price of dementia in later life?

Sleep-learning is a brave new world that we’ve just begun to explore. But one thing is clear: “night school” is about to take on a whole new meaning.


Image credit: Shutterstock.com

Shelly Fan

Shelly Xuelai Fan is a neuroscientist at the University of California, San Francisco, where she studies ways to make old brains young again. In addition to research, she's also an avid science writer with an insatiable obsession with biotech, AI and all things neuro. She spends her spare time kayaking, bike camping and getting lost in the woods.

Discussion — 10 Responses

  • rtryon March 20, 2016 on 10:56 am

    At a very early age (I’m now almost 84) I noticed that my mind wanted to keep track of my physical orientation to make it easier to get home. Moving around through different states and living places seemed to reinforce this semi-automatic skill and its still hard to get me disoriented to N-S or E-W directions even when moving through large buildings with halls that twist and turn and finally come to a window looking out and I can usually point out the direction! It is not surprising that I can much later recall how to get to this same spot!

    Does it mean my sleep includes all sorts of mental housekeeping activity, as it does, that includes re-inforcing these travel experiences? To be sure a compass helps but not even winding roads keep me from observing Sun or Moon or other inertial navigation memories.

    So, I support your notions without offering proof.

    • richemon rtryon March 20, 2016 on 4:06 pm

      That’s pretty cool, nearly 84 and fully subscribed to the online singularity movement. I can only imagine what this line of reasoning will do to kids and teenagers who pick it up, they’ve never known a world without WiFi.

      • DSM richemon March 20, 2016 on 5:18 pm

        I personally educate my kids in the basics of celestial navigation, it is a fun way to learn geometry and trigonometry, not to mention a great introduction to an important part of the history of science and technology.

  • DSM March 20, 2016 on 2:16 pm

    I think that loading up with structured stimulus and then immediately having a nap is a safer and more effective strategy, otherwise perhaps your sleep learning should start about 6 hours after you fall asleep and not at the start of the night.

    One interesting implication of sleep learning is that it also implies that the sensory pollution in our environment will impact on the functioning of our brains and that if sleep learning compromises brain house-keeping then so does a sub optimal sleeping environment.

    • richemon DSM March 20, 2016 on 4:19 pm

      I agree with a power nap after a major info download.

      City living is sensory pollution. There’s an amazing clarity one can experience in nature, suddenly the world stops and there’s a silent conversation, sorely lacking in modern life.

      • DSM richemon March 20, 2016 on 5:22 pm

        The best sleep I ever had was when I lived in a place called Darkwood, it was an isolated river valley and as well as being very quiet, particularly in winter on a still night, the woods were indeed very dark. There used to be a Buddhist retreat in the area too.

  • almostvoid March 21, 2016 on 2:33 am

    CG Jung had the finger on the pulse: dreaming is an alchemical process of the soul sorting out its destiny which is represented in symbolic forms. You need to study the visuals in alchemy and having read several of his books I now find -sometimes- my dreams are in alchemical transmutation. Using dreams for solving problems is all very nice but this inner journey leads to a balanced psyche and that is more important for one’s total beingness. Pity scientists are lead so easily up the -follow the dollar=research=ideological pursuit.

  • Kazunori Saito March 23, 2016 on 5:20 am

    Still not too late. I can teach you Japanese. haha

  • Ralphoo March 28, 2016 on 8:50 pm

    “Learning by osmosis might not be such a crazy idea after all.”

    What other way is there to learn? Awake or asleep, our brains are working. They just naturally build skills and memories. Our job is to take good care of them.