Imagine you’re cleaning off your desk. You sort some papers into folders with the relevant labels. Others you red-tag as “urgent” or yellow-tag as “semi-urgent.” Quite a few go directly into the large circular file at your feet, also known as the trash basket.
Turns out, it seems that your brain does something very similar with your memories every night as you sleep.
The journal Nature Neuroscience has just published a special issue on memory, and among its authors is Dr. Robert Stickgold of Beth Israel Deaconess Medical Center and Harvard Medical School, a leading researcher on the role that sleep plays in consolidating memories.
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In the past, my layperson’s take-home message from his complex research might have been, “Sleep helps strengthen memories. So if it’s the night before an exam and you have the option of cramming more or sleeping, better to sleep.”
But there’s ever so much more to know, and for researchers to find out — about sleep states beyond REM, about how our brains “tag” some memories for retention and dump others, about how sleep can improve performance overnight — even about why toddlers so desperately need naps.
Our conversation, lightly edited:
You’ve written a sweeping review of years of recent research on what sleep does to memories. How would you sum up for a lay audience what we now know?
I’d start by telling them a true story, which is that about 50 years ago, my father commented to me that when he was in law school studying for an exam, he would stay up late at night reading case after case, and go to sleep with a complete mishmash of cases in his mind. When he woke up the next morning, they had just all been filed away in the right spot. That was 50 years ago, and I can now say, ‘Yes, and now we have an idea how.’
It really does happen while you sleep, and although some of it can happen while you’re awake, especially if you’re consciously working at it, sleep seems to be a time that’s been set aside to make sure that filing gets done, even without your awareness or intent.
So sleep is a time of sorting and discarding memories?
Sleep is doing about five things. The first things we knew that it could do is strengthen some memories, so you were just better at them in the morning. It could stabilize some memories so they aren’t as susceptible to interference. Those are the first pieces we got, between about 2000-2007.
And then we started to get into the more subtle stuff. Sleep can also help you hold onto some of the things you learn during the day, and let the rest be forgotten. And that’s the concept of selective retention.
And there is now evidence that in fact it doesn’t just let the rest of it passively decay or be forgotten, but it can help it along that path of being forgotten.
Then, it turns out that the brain also does a lot of smarter stuff while you’re sleeping. If you learn a list of words, it can extract the gist from them. It can make it so that you’re two and a half times as likely to discover an insight into an alternative way of, say, solving math problems.
It can extract patterns. Say I’m trying to teach you how to use a deck of cards to predict the weather, which isn’t very obvious. Imagine you’ve got four aces and in 200 trials, I show you one, two or three of the aces and then tell you it turned out to be rainy or sunny. And it turns out that it’s not a simple pattern, because the cards only have a probability of being associated with sun or rain, so any given card, or two or three cards, predict sun or rain more or less, but not one or the other with certainty. Over 200 trials, people get to the point that they can predict with 70-to-80-percent accuracy what we’re going to say the weather is. But after a night of sleep they’re 10 percent better. If they don’t sleep, they get no better.