Study discovers how one sleep stage reactivates memories

How the brain decides what to store and what to forget.
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Sleep feels great. After a long day, there is nothing quite so welcoming as a warm, comfortable bed. We hardly need science to tell us that everything feels a little harder, tasks take a little longer, and everyone seems a little more annoying when we have not had enough sleep. For those of us who have gone days without sleep, the world becomes hazy and surreal, seen through the febrile, distorted mind of the sleepless.

And while we are still not entirely certain what sleep does, or why we seemingly need so much of it, neuroscientists have discovered that a good night’s sleep is essential to build and store our memories. What’s more, according to a new paper by Virginie Sterpenich et al., published in Nature Communications, we now know what type of memories our brains prefer to store, too.

Delta waves build memories

When we sleep, lots of things happen. Our body temperature drops, our heart rate slows, and our brain cycles through different types of waves. We begin with alpha waves, which are responsible for our awareness. Then we move into theta waves, which mark that slightly unconscious borderland between being awake and asleep. (Theta waves also appear in deep relaxation and meditation.) Finally, we move into the deep, dreamless sleep of delta waves. It is in this final stage that the brain does one hugely important thing: it builds memories.

Sweet dreams might not be all that they are cracked up to be.

During non-rapid eye movement (NREM) sleep — that is, our dreamless, delta wave stage — our brains carefully transfer what we have learned or experienced recently from our short-term to our long-term memory systems. The same parts of our brain that are involved in waking experiences are reactivated during NREM, all to optimize memory consolidation. It is a bit like replaying or revising an experience over and over, all while we are none the wiser.

As Matthew Walker writes in his book Why We Sleep, when we enter this NREM stage, we “manage the finite storage space within the brain,” and our brains decide which “memories are fresh and salient, and which memories that currently exist are overlapping, redundant, or simply no longer relevant.” In short, our brains are choosing for us what it is that we should remember. (It is also why a good night’s sleep before an exam is likely better for you than an all-night cramming session).

Which memories do our brains choose to store?

One of the more interesting questions from this is: how and why does the brain select memories to reprocess during sleep? Why does the brain choose to remember a song lyric but not a colleague’s name? This is where Sterpenich’s aforementioned paper comes in.

The first observation is that memories are more likely to be stored if they potentially aid an individual’s survival. It makes sense, then, for the brain to store as memory those experiences that have a greater positive or negative emotional association — what is called “affective relevance” — because it will help guide our future behavior.

For instance, I am more likely to remember a painful burn than a red car passing by because the former is much more useful to my future life — that is, do not touch hot pans. Similarly, song lyrics are fun, so we remember them. Gerry from IT isn’t so fun, so we forget his name.

Notably, our brains prefer to store positive and rewarding experiences. The team at the University of Geneva had participants play two games, some of which they won. The results showed that people were much likelier to remember the details of the games that gave them a rewarding experience of victory. It seems that when we have pleasurable experiences, the parts of our brain responsible for the feeling of reward — the hippocampus and the ventro-temporal-limbic system – are reactivated during our night’s sleep, and so our brains store these experiences as long-term memories. In short, if something feels good, we are more likely to remember it.

This has potentially useful implications for learning and memorizing information. If we want to remember something, we are much more likely to do so if we associate it with a positive experience. So, if you want to remember an important date, eat some chocolate as you look at it.

Dreaming bad things

But what about all those bad experiences and the tendency we all have to remember the negative things in life? While the paper indicates that positive experiences are more likely to be stored during our deep NREM stage of sleep, it leaves us with the question about the role of sleep on our negative memories.

There is a body of evidence that suggests that our negative experiences are reactivated during the REM stage of our sleep cycle. When we are dreaming (which almost exclusively happens during the REM stages of our sleep cycle), we are more likely to be storing negative experiences as memories.

The study did not have the dataset to prove this either way (their participants did not enter REM stages long enough to measure it), but this suggests that when you have a night of long and intense dreaming, you might also just have put to memory all those sad, scary, or disappointing moments of your day. Sweet dreams might not be all that they are cracked up to be.

Jonny Thomson teaches philosophy in Oxford. He runs a popular Instagram account called Mini Philosophy (@philosophyminis). His first book is Mini Philosophy: A Small Book of Big Ideas.

This article was reprinted with permission of Big Think, where it was originally published.

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