The "Swiss Army Knife" Of Health: A Good Night's Sleep

Nov 14, 2017
Originally published on November 13, 2017 8:00 pm

What does the song "Satisfaction" by The Rolling Stones have in common with the periodic table of elements? Both are the product of dreams.

Creative reawakening is just one of many boons provided by a full nights rest, argues neuroscientist Matthew Walker.

"Sleep is the Swiss army knife of health. When sleep is deficient, there is sickness and disease. And when sleep is abundant, there is vitality and health."

On this week's episode of Hidden Brain, the mind and body benefits of eight full hours of sleep — and yes, Matthew Walker says that's how many hours of rest you should be getting.

"If we didn't need eight hours of sleep and could survive on six, Mother Nature would have done away with 25 percent of our sleep time millions of years ago. Because when you think about it, sleep is an idiotic thing to do. If sleep does not provide a remarkable set of benefits, then it's the biggest mistake the evolutionary process has ever made."

Hidden Brain is hosted by Shankar Vedantam and produced by Maggie Penman, Jennifer Schmidt, Rhaina Cohen, Parth Shah, and Renee Klahr. Our supervising producer is Tara Boyle. You can also follow us on Twitter @hiddenbrain, and listen for Hidden Brain stories each week on your local public radio station.

Copyright 2017 NPR. To see more, visit http://www.npr.org/.

SHANKAR VEDANTAM, HOST:

This is HIDDEN BRAIN. I'm Shankar Vedantam. Last week on the show, we featured the story of Randy Gardner, a San Diego man who went 11 days without sleeping, breaking a world record.

(SOUNDBITE OF ARCHIVED BROADCAST)

RANDY GARDNER: You don't need sleep. That was the thinking back in the '60s, and that's the thinking that I had. Of course, as it turns out, that was absolutely wrong.

VEDANTAM: Randy's exploit would come back to haunt him later on in life in the form of crippling insomnia. This week on the show, we continue to look at sleep and explore one of the greatest mysteries in human behavior. Nature has endowed us with an amazing brain. So why in the world would nature have that very same brain put itself to sleep for one-third of our lives?

MATTHEW WALKER: If we didn't need eight hours of sleep and we could survive on six, Mother Nature would have done away with 25 percent of our sleep time millions of years ago because when you think about it, sleep is an idiotic thing to do. If sleep does not provide a remarkable set of benefits, then it's the biggest mistake the evolutionary process has ever made.

(SOUNDBITE OF MUSIC)

VEDANTAM: Matthew Walker is a professor of neuroscience and psychology at the University of California at Berkeley. He studies sleep, and he's the author of the book "Why We Sleep." I started our conversation by asking him to tell me a story he describes in his book. It's about a pianist who relied on sleep for his creative process.

WALKER: Yeah, I was giving a public lecture on sleep. And at the time, we didn't know too much about sleep's role in learning and memory, of which we now know a great deal. And this wonderful sort of distinguished-looking gentleman with a fantastic kindly face walked to me. He was dressed in this great sort of tweed suit. And he said, I'm a pianist.

And I was fascinated by what you were saying about sleep and how active a brain state it is. And I wanted to tell you that there are times when I will be trying to learn a new piece, and I just can't get it. And I get frustrated, I make the same mistake at the same place each and every time. And I'll sometimes play late into the evening. And I will walk away continuing to be frustrated, have a night of sleep and then when I come back and I sit down the next morning, I can just play perfectly.

(SOUNDBITE OF PIANO)

WALKER: And what he was suggesting, perhaps, was that it wasn't practice that made perfect, it was practice with a night of sleep that made perfect.

(SOUNDBITE OF PIANO)

VEDANTAM: I want to run two other examples by you, both of which seem to suggest that remarkable things happened to us while we're sleeping. I understand that guitarist Keith Richards from Rolling Stones kept his instrument and a recorder by his bed, and he did it in case inspiration struck while he was asleep. Did it ever happen?

WALKER: It did. And it's, I think, one of the wonderful stories of sleep-inspired creativity, of which there are many. He would have this tape recorder and he would have his guitar and who knows what else around in the bedroom at the time. And one morning, he woke up and the tape had recorded all the way to the end. And he didn't remember anything about that night.

So he rewound the tape and he played it. And he says - and this is in his autobiography - there was almost this sort of ghostly vision of him strumming the chords to "Satisfaction."

(SOUNDBITE OF THE ROLLING STONES' SONG, "(I CAN'T GET NO) SATISFACTION")

WALKER: Arguably the most popular Rolling Stones song ever. And he said he created that classic guitar riff from his sleep.

(SOUNDBITE OF SONG, "(I CAN'T GET NO) SATISFACTION")

THE ROLLING STONES: (Singing) I can't get no satisfaction.

WALKER: It was a dream-inspired musical piece of creativity followed by about 42 minutes of snoring.

(LAUGHTER)

WALKER: Lots of scientific discoveries though to have been birthed by way of dream-sleep-inspired creativity.

VEDANTAM: Speaking of scientific discoveries, I understand there's a connection between sleep and the discovery of the periodic table.

WALKER: Dmitri Mendeleev was trying to understand how all of the known elements in the universe fit together. And it was his obsession for years. And he struggled and he couldn't figure it out. He would create playing cards with all of the different elements and he would deal them to see if he could find some equation by way of which they all fit together.

And apparently so the story goes, February 17, 1869, he fell asleep exhausted, frustrated - couldn't figure it out. And that, in his sleeping brain, emerged the solution that his waking brain could not divine. He started to realize how all of these swirling elemental ingredients could actually snap together in this sort of what he described as a divine grid where you had each row, each period and each column, each group having this logical progression of atomic and orbiting electron characteristics. And he woke up and he penned down this remarkable table, the table that we now call the Periodic Table of Elements. And he noted that he made just perhaps one or two changes. And that document still exists.

You can still see that dream-inspired piece of creativity. So, you know, there is one of the greatest, I think, arguable problems to solve. And it wasn't a waking brain that solved it. It was a sleeping and a dreaming brain that did it.

VEDANTAM: Matthew says there are two types of sleep. Rapid eye movement sleep, also known as REM - this is when we dream. Then there's non-rapid eye movement sleep or non-REM.

WALKER: And those two types of sleep actually play out in this wonderful battle for brain domination throughout the night. And that cerebral war is won and lost every 90 minutes and then replayed every 90 minutes to produce what we call a sleep cycle. And you go down into non-REM sleep first and then you go up into REM sleep and then you repeat the cycle.

VEDANTAM: You say that non-REM sleep might be implicated or involved in cementing memories and that there's a popular song that might get at this idea.

(SOUNDBITE OF SONG, "THE SOUND OF SILENCE")

SIMON AND GARFUNKEL: (Singing) Hello, darkness, my old friend. I've come to talk with you again because a vision softly creeping left its seeds while I was sleeping and the vision that was planted in my brain still remains.

VEDANTAM: So Matthew, what do Simon and Garfunkel get right about non-REM sleep?

WALKER: It is prophetic wisdom of the most remarkable kind. You know, I think that they were suggesting things there that were 20, 30 years ahead of their time. We now know that we imprint information during the day. We sort of - that seed is planted there within the brain during the day. In other words, we learn information. But we also know that that vision that was planted in the brain still remains in the sound of silence, in this - in the dark of night.

And it's there that specifically deep non-REM sleep goes to perform its memory functions. Deep non-REM sleep almost hits the save button on those recently acquired informational pieces so that when you wake up the next morning, you have remembering rather than forgetting.

VEDANTAM: Non-REM is all about helping us retain information. And as we saw from the music of The Rolling Stones and the creation of the periodic table, REM sleep, dream sleep, spurs creativity. But Matthew says dreams also have another function.

WALKER: The second function, however, is very different. That function seems to be about emotional therapy or what I would describe as overnight therapy. Dream sleep provides a fascinating neurochemical soothing balm. It is during dream sleep and only during dream sleep when our brain shuts off a stress-related neurochemical called noradrenalin.

Now, its sister chemical everyone will be familiar with in the body. That's called adrenaline. But the version of it upstairs in the brain is called noradrenalin. And it's during dream sleep that that chemical is actually shut off. But what we also know is that the emotional and memory centers of the brain during dream sleep light up in terms of their activity. And so we've proposed that dream sleep provides this perfect opportunity where we can start to reactivate and replay painful, difficult emotional experiences.

But we do so in a neurochemically, quote, unquote, "safe" environment. And we now understand that dream sleep actually helps separate and strip away that painful emotional sting from those informational experiences so that you wake up feeling better about it. You wake up with a memory of an emotional event but it's no longer emotional itself. It's that form of nocturnal therapy.

VEDANTAM: But what about bad dreams?

GRETA PITTENGER, BYLINE: Hi, I'm Greta Pittenger.

VEDANTAM: Greta is a researcher at NPR. And about eight or nine years ago, her relationship with sleep changed. I'll let her tell you the story.

PITTENGER: I had been dating a guy who is now my husband. But at the time, we were just dating. He rode a motorcycle, and that was his only form of transportation. So whenever I'd go out with him or he'd drive me to work or something, I would ride on the back and had my own helmet and stuff. Late one night, we were coming home from a party and we were in downtown Seattle.

Streets were pretty empty, but we were going pretty slow coming out of a stoplight and a large white SUV ran a red light and crashed into us from the left side.

(SOUNDBITE OF CAR CRASH)

PITTENGER: The next thing I remember is waking up on my back in, like, an elevated planter kind of off on the side of the sidewalk. You know, there's, like, the street on one side and then the other side has, like, a little kind of retaining wall with, like, plants and ferns. And so I was kind of halfway into that. It was about - I'd fallen off the bike and hit my right thigh on that concrete planter and broke it - broke the femur, like, right in half. And when I woke up, I was on my back with my legs kind of dangling and there was flames around me. And then, like, that's when I think I saw my then boyfriend Joey (ph) on the sidewalk.

And it was, like, he was also injured separately but didn't have much time to think about any of those things 'cause luckily the police station was two blocks away. And so then police were the first people there and they came and dragged us out.

(SOUNDBITE OF AMBULANCE SIREN)

PITTENGER: The actual crash of the car had punctured the motorcycle. And then there were - the first sound I really remember was an explosion of the motorcycle tank blowing up, which was - yeah. I don't trust my memories of how intense it was, but it was very, very scary. Joey actually broke the other femur. He broke his left femur from the impact of the car.

I broke my right femur from hitting that wall. He also broke a few other bones, so I stayed in the hospital for maybe four days after surgery. And he stayed for I want to say at least a full week, maybe 10 days 'cause he had a couple surgeries. I didn't sleep pretty much at all, partially because of the painkillers.

But I think also that's when, like, bad dreams were starting. And I couldn't stop reliving that moment of the crash. I'm not sure if I had these exact dreams in the hospital. But for a couple years after all of this, I would just keep having dreams about things crashing into me or things running over me and then specifically that moment also when that car hit us.

I would keep going back to that. And I remember waking up, like, with a start - you know, like, when you wake up suddenly and sometimes you can't remember why but you know you feel like you fell. Like, you feel like you just fell into bed, that feeling. I like to write, and I've kept a journal since I was in fourth grade. I had written about it right after it happened at my mom's insistence.

I was staying with her and my dad at the time and just, like, I'd wake up from my nap just bawling. She'd said, like, you know, maybe you should just write it down and let it go, kind of hoping that might help. And I think it did for a little bit, but then it kept coming back. And I tried to avoid it.

VEDANTAM: When we were talking about dreams some time ago, Matthew, you said that one of the potential virtues of dreams is that they might allow us to relive or experience things that happened to us or things that might happen to us in a relatively safe space. And as we do this, we process what happened and then potentially learn from it. What about nightmares, though?

Nightmares are not pleasant. They can be disruptive. They can actually be acutely painful and certainly in Greta's case, they were disruptive to her life. Why would the brain be designed to have nightmares?

WALKER: It's not clear whether the brain actually is designed to have nightmares or whether this is actually the process going awry. And we think it may be the latter because when we look at patients, for example, who have post-traumatic stress disorder or PTSD, repetitive nightmares are actually so reliable in those patients that they actually form part of the diagnostic criteria for that disorder itself.

So there is something strange that we've yet to fully understand about particularly emotionally intense trauma dreams. What we think is happening in the case of PTSD is that that chemical that we spoke about that normally is shut off during dream sleep, the chemical noradrenalin, remains too high. And it may be that when that chemical is too high in its concentration, we can't gift ourselves that normal therapeutic benefit that REM sleep provides so that the dreams themselves become particularly emotionally strong and difficult.

And you don't get that resolution the next day. And so the process steps and repeats. And it happens time and time again. And it is perhaps only when there is some degree of contextualization, be it by way of medication that is now given to certain PTSD war veterans. For example, drugs out there that seem to help lower that chemical, that stress-related chemical, give them normal dream sleep.

And it gives them back that ability to process those events. That's one way to help. Another is that perhaps by journaling it and going through that process of shifting the context or reformulating it in one's mind, it becomes less stressful.

VEDANTAM: In fact, this is exactly what Greta discovered herself, too.

PITTENGER: So about two or three years after the crash, I was finally seeing a psychiatrist about the kind of lasting trauma and trying to get over that and trying to sleep - I mean, I was trying to help my insomnia as well. And we talked about those dreams. And he suggested - he knew I liked to write. And so he suggested in my journal just rewriting that dream because it doesn't - it's a dream.

It's not reality. And so it doesn't need to mimic reality. So just change it. The way I changed that dream was that instead of a car coming and hitting the motorcycle, the motorcycle transformed into a winged horse and flew away - away from the car, away from all that crash, and then just landed safely back at our apartment, dropped us off. And so writing about it again years later, but in a totally different way, not trying to be accurate, not trying to remember was much different. It definitely got me to stop having those vivid dreams. I've had flying dreams in the past, but I started to have more types of those dreams of the kind of being lifted away from gravity and from the weight of these emotions. You know, like, when you wake up from a dream, and it doesn't really leave a mark on you? That was a good feeling to kind of be like, oh, yeah, I guess I did have that dream. Oh, yeah. That was nice.

Just, like, this past month, Joey and I bought mopeds, which is, like, something he used to do before he even had a motorcycle, and I have never ridden before. And I'm, like, I feel like such a badass on this, like, little, like - it's such a dorky little thing. It's, like, half bike with a tiny engine on it, and it sounds like super high-pitched and has the dorkiest horn, but it's, like, yeah, look how cool I am. I have to wear a full-face helmet. (Laughter).

VEDANTAM: When we come back, we look at the amazing range of things a good night's sleep can accomplish.

WALKER: You know, sleep is the Swiss Army knife of health.

(SOUNDBITE OF MUSIC)

VEDANTAM: So let's talk just for a bit about the amount of sleep that people need. You and many other experts say people should strive to get eight hours of sleep every night. To tell you the truth, I got about six hours last night, and I feel fine.

WALKER: Tell me where you live. I am coming round tonight. We will have a sleep salon, a sleep - I will inflict change no matter what.

VEDANTAM: Well, here's my question, Matthew. If I can get away with sleeping 25 percent less than the recommended amount one night, why can't I do it every night? And just think of the upside, you know? I can spend two hours every day reading wonderful books like yours, building a better podcast, being more productive. Surely, it's a good thing.

WALKER: If we didn't need eight hours of sleep and we could survive on six, Mother Nature would have done away with 25 percent of our sleep time millions of years ago. Because when you think about it sleep is an idiotic thing to do. You're not finding a mate, you're not reproducing, you're not finding food, you're not caring for your young. Worse still, you're vulnerable. So on any one of those counts, it should have been excised from the evolutionary process. So as it has been said before, if sleep does not provide a remarkable set of benefits then it's the biggest mistake the evolutionary process has ever made. And it didn't make a spectacular blunder in putting in place through 3.6 million years of evolution this thing called an eight-hour sleep need.

So I know that you think you're OK with six hours of sleep, but trust me, you're not. And we know this to be true that your subjective sense of how well you're doing on insufficient sleep is a miserable predictor of objectively how you're doing with too little sleep. So it's a little bit like a drunk driver at a bar who picks up their keys after, you know, six or seven shots and says, I'm fine to drive. Off I go. And your response is, no, I know that you think you're fine to drive, subjectively, but objectively, trust me, you're not. You're impaired. It's the same case with insufficient sleep, too, and I think it's one of the reasons that we often underestimate how damaging a lack of sleep is on our health and our wellness and our cognitive function.

VEDANTAM: You know, I remember once going to a talk that showed the level of light pollution on the planet, the parts of the planet where there were the highest levels of artificial light. And then the researcher took that, you know, the image off the screen and replaced it with an image showing the distribution of prostate cancer around the world. And there was a remarkable correlation between the areas of the world which have light pollution, where presumably people are staying up later and later at night and presumably getting less sleep than they need, and the incidence of prostate cancer. Now, of course this is a correlation. We don't know if one is connected to the other. But you said there has been some evidence at least that sleep might be implicated in the development of cancer.

WALKER: There is. And it's fast becoming, I think, strong evidence and causal as well. We know, for example, that one single night of short sleep - and these are laboratory studies where you perhaps are limited to just four hours of sleep for one single night - the next day that will drop critical anti-cancer fighting cells, called natural killer cells, by 70 percent, seven zero. That is an alarming state of immune deficiency, and it happens quickly after essentially just one bad night of sleep. We also know from the associational evidence that insufficient sleep is linked to cancer of the bowel, cancer of the prostate and cancer of the breast. And the link between a lack of sleep and cancer has since become so strong that the World Health Organization has now classified any form of nighttime shiftwork as a probable carcinogen. In other words, jobs that may induce cancer because of a disruption of your sleep-wake rhythms.

(SOUNDBITE OF ARCHIVED RECORDING)

UNIDENTIFIED MAN: (Speaking Danish).

WALKER: Denmark, based on the strength of the evidence that we were just discussing, became the first country to actually pay worker compensation to women who had developed breast cancer after years of nighttime shift-working government sponsored jobs. There are clearly some professions where we do want and need people to work those shifts. You know, if I have a burst appendix at 3 o'clock in the morning, I'm so grateful to people at the hospital who will see to me and save my life. So I think automation is going to help, I think, with this revolution of technology where we can start to limit that type of shift work whenever possible. We should absolutely do that and start to scale it back. We can also architect professions better, I think. We know that people are genetically predisposed to being nighttime people or morning people. So why aren't we asking those questions and seeing if the people who would much prefer what we call the owls, who like to sort of go to bed very late and wake up very late, why don't we think about asking those questions and seeing if we can help people sort of fit what we call their chronotype, which is their morning-ness or evening-ness propensity, fit that into the job flexibility in those work hours and see if there's some overlap?

VEDANTAM: I understand that before you came to Berkeley you taught at Harvard, and you once stirred a bit of controversy among the faculty when you wrote an op-ed for The Harvard Crimson. What did you recommend in the op-ed?

WALKER: Well, I was asked, based on the findings that we were publishing regarding how important sleep is for memory, to write a piece on this. And I ended up, rather than describing the science, actually going after the structure of education itself because what we tend to do at higher education institutes is have a long term or a semester, and then we end-load that semester or that quarter with all of these finals where people have to cram vast amounts of information into the brain. And what do they do? They end up pulling the all-nighter. And we've done the studies, and it's very clear. If you pull the all-nighter, you're about 40 percent worse at cramming facts into the brain. So that behavior is contrary to a sound education. And I went to task and I said that in fact this was not the students' fault, it was our fault, educators, because we are designing a system that actually co-ops and encourages that type of behavior, pulling the all-nighter. So if our goal as educators truly is to educate - and, P.S., not risk lives in the process - then I think we were failing our students with this model of end-loading school semesters or university semesters with all of these exams.

VEDANTAM: When you were a kid, your family took a vacation to Greece. And then when you visited Greece again as an adult, you noticed a very big change. What was it?

WALKER: Back in the 1980s when I went on holiday there, were signs in the shop store windows that would give the opening hours. And they would open from between sort of 10 to 2 and then it said closed between 2 to 4 or 2 to 5 p.m., and then open from 5 through until 10:00 or 11:00 in the evening. And it was so different to the way in which sort of shops back in England would operate. You know, maybe there was a one-hour lunch break or a half-hour lunch break, but for the most part it was 9 to 5 hours, classic. And of course what it was describing was this classic siesta-like behavior.

Now, back in the mid 1990s, the Greek culture actually started to abandon the siesta-like practice. And fortunately, or, unfortunately, a group of scientists from Harvard University School of Public Health decided to quantify the health consequences of this radical change in sleep practice. And with many Greek tragedies, as was the case here, the results were heartbreaking, but in the most literal sense. What they actually observed was a 37 percent increase risk for death from heart attacks across that six-year period as a consequence of doing away with that siesta behavior. It was actually particularly strong in working males, almost a 60 percent increased risk of death from heart attacks.

(SOUNDBITE OF MUSIC)

WALKER: So I think that that again suggests not only how important sleep is, and when sleep is taken away we see this type of danger to our cardiovascular health. It raises actually a different question, which is, how should we be sleeping? And are we sleeping currently - the way in which we do, through modernity in industrialized nations - in the way that we were designed? The answer may actually be no because the way that we currently try to sleep is what's called monophasic sleep where we sleep one single bout throughout the night. We aim for how ever much we can. But if you look at some cultures who are untouched by electricity, sort of hunter-gatherer tribes for example, they actually tend to sleep bi-phasically. They tend to sleep for sort of six, seven hours at night then they'll have a siesta-like nap in the afternoon. And it turns out that we all have this in us. It is genetically hardwired that we all have a preprogramed drop in our alertness some time after lunch.

Now, many of us think it has to do with the lunch that we have. It's actually not. You can stop the lunch and you still get it. It's this sort of, you know, you've seen those boardroom meetings, you know, in the afternoons where you get lots of that head nodding that starts to happen. You know? It's not people listening to good music. it's actually people giving way to this hardwired, programmed drop in our alertness, which actually does argue from an evolutionary perspective that we should be sleeping bi-phasically rather than monophasically - two bouts of sleep rather than one, perhaps.

VEDANTAM: I have to say, Matthew, that in some ways when it comes to sleep, there's almost a sense of people bragging about not getting enough sleep, right? I mean, it's certainly, in the United States it's seen as a, you know, badge of honor to say, you know, I get very little sleep because I'm so productive and I work so hard and I achieve so much.

WALKER: You're so right. Because what we've done is actually stigmatized sleep. Sleep has an image problem right now, and it's not just in America. You know? And we do. We label people who get sufficient sleep - and I choose that word very carefully - with being lazy, with being slothful. And that is a terrible disservice to society. And we don't always have that opinion, by the way. You know, no one looks at an infant sleeping during the day and says, what a lazy baby.

VEDANTAM: (Laughter).

WALKER: And we don't. You know, and we laugh, but we don't because we know that sleep at that time of life is non-negotiable. It's absolutely necessary. But now even into early childhood, not only do we abandon the notion that sleep is important and should be celebrated, we chastise people for getting sufficient sleep and give them this label.

VEDANTAM: Tell me about your own sleep habits, and tell me what you do to ensure you get a good night's rest.

WALKER: I'm going to sound like a desperate prude, and I'm so sorry. And it sounds hokey, as well, but I actually give myself a non-negotiable eight-hour sleep opportunity every night. If you knew the evidence as I do, you would - trust me, you wouldn't do anything different. You know, sleep is the Swiss Army knife of health. You know, when sleep is deficient, there is sickness and disease. And when sleep is abundant, there is vitality and health. My family, for example, has a history of cardiovascular disease. Sleep is a wonderful form of nighttime blood pressure medication. Why would I skip on it? Why would I try to give way to the genes that I've been given in this genetic lottery and succumb to cardiovascular disease? So I have to say, I'm sorry, I do practice what I preach.

VEDANTAM: Do you try and stick to very rigid hours - sleeping at the same time, waking up at the same time? Do you use an alarm clock? Do you avoid technology before you go to sleep?

WALKER: I do. So I stop checking my email at a certain time. I have software installed on my computers that does away with the harmful blue light, and I shut them off at least an hour and a half before bed. But you mentioned perhaps the single most important sleep prescription that I could give everyone which is regularity. Just go to bed at the same time, wake up at the same time, no matter what. Whether it's the weekday or the weekend, if you've had a good night of sleep or a bad night of sleep, stay as regular as you can. That's the best piece of advice I can give you for getting good sleep at night.

VEDANTAM: Matthew Walker is a professor of neuroscience and psychology at the University of California at Berkeley. He studies sleep, and he's the author of the book "Why We Sleep." Matthew, thank you for joining me today on Hidden Brain.

WALKER: You're very welcome. Thank you very much, and I do hope you sleep well tonight.

(SOUNDBITE OF MUSIC)

VEDANTAM: This episode of Hidden Brain was produced by Parth Shah. Our team includes Tara Boyle, Jenny Schmidt, Maggie Penman, Renee Klahr and Rhaina Cohen. Our unsung hero this week is Holly Herzfeld. Holly is an intern at NPR, and like many of the multi-talented interns who come to us, Holly happens to have a special skill. She plays the piano. When we wanted someone to play a piano riff for us at the start of this episode, we turned to her. Thanks so much, Holly. For more Hidden Brain, you can follow us on Facebook, Twitter and Instagram, and listen for my stories on your local public radio station. If you haven't subscribed to the podcast, please do. And if you've never told a friend about Hidden Brain, try it and see how much more they love you.

(SOUNDBITE OF MUSIC)

VEDANTAM: I'm Shankar Vedantam, and this is NPR. I hope you have sweet dreams tonight. Transcript provided by NPR, Copyright NPR.