We know that sleep helps us recall and learn information and ideas. But a lot happens (and stops happening) during rest. And some of us wake up from naps with greater gains in wisdom, wit and a-ha moments than others. So, it's hard to pinpoint exactly how sleeping supports cognition. Most efforts to clear the fog concern a specific stage of sleep or pattern of brain activity. Some studies hammer home the importance of dreaming to REMember. Others urge going hard on slow-wave Zzzs or zero in on neural networks that go gangbusters during shuteye.
In any case, attempts to understand sleeping-for-smarts center on the brain. Seemingly, this brain-centric approach makes sense — because thinking is the brain's domain. But, what seems sensible may be shortsighted. In a recent study, published in PNAS, researchers at UC Riverside link other bodily processes to nap-related cognitive improvements. They specifically looked at the role of the autonomic nervous system (ANS), which controls involuntary functions such as heartbeat, breathing and digestion.
"I think that people have been ignoring what happens from the neck down," said lead study author Sara Mednick, a psychologist at the University of California, Riverside. "[This study] really opens up a whole other field of research, focused on looking beyond the brain to understand how sleep affects memory."
Mednick herself began looking beyond the brain after completing a 2009 study on napping and creative insight. She and colleagues had found that naps involving REM sleep corresponded to improved results on the Remote Association Test (RAT), a standard psychological measure of creativity.
On the test, participants were shown a group of three words (e.g., oven, pants, rod) and had to come up with a fourth word, linking the other three. Participants took the test in the morning, took naps (well, some only got quiet time) and re-took the test in the afternoon. Half the nappers took REM-inclusive naps while the other half took naps sans-REM, thanks to REM suppressing drugs. The nappers did, on average, 40 percent better on the re-test than the morning test, whereas the other two groups didn't do any better the second time around.
After finding a link between REM-enriched naps and improved RAT scores, Mednick began wondering about the root of the link. Rodent studies tipped her off to the importance of the autonomic nervous system. "I'd never really considered [it] in terms of what the brain is doing," said Mednick. "You don’t really think, 'Well, what is the body doing while the brain is doing its thing?'"
But the scientific literature was persuasive. Previous research had shown that manipulating peripheral nerve activity in animals changed memory consolidation. (Remember, the brain and spinal cord make up the central nervous system (CNS). The body's other nerves belong to the peripheral nervous system, of which the ANS is one branch.)
"So, the question is," said Mednick, "If all these signals are coming up through the body to the brain and we know that activity in the vagus nerve is actually very high during sleep, is there a relationship between all of this sleep-dependent memory stuff that we’re looking at and the autonomic nervous system activity that we know occurs during sleep as well?"
And the answer, she believes, is yes. To figure this out, Mednick and colleagues re-ran a spiffed up version of their 2009 study. Same test, same naps and, luckily, same (or similar enough) results with regard to am-versus-pm performance. But, in addition to measuring electrical brain waves with EEG, researchers measured heart activity using ECG. Specifically, they measured heart rate variability. High variability is good; it indicates steady, health cardiac function. Low variability translates to an overworked ticker. And, studies have connected heart-rate variability to cognitive performance — but they've only done so for waking subjects. No one had tried to fit dozing into the picture.
They looked at test-performance improvements that hinged on sleep, meaning cases where napping conditions were the only difference between participants whose scores did and didn't improve. From analyzing sleep alone, researchers could partially explain changes in performance. But, they were able to predict improvement in scores far more easily when they had information about participants' sleep as well as their heart rate variability. Combined, sleep and heart data explained 73 percent of morning-to-afternoon changes. Mednick wasn't expecting such pronounced results.
Why would the heart be so relevant to matters of the head? Right now, their money's on the vagus nerve, a big-shot cranial nerve that runs from the brainstem to the tummy and goes wild during sleep. Among other duties, the vagus nerve sends messages about respiration, heart function and gut activity to the brain. Those messages, of course, come in the form of brain chemicals, or neurotransmitters. When the vagus nerve is active, the neurotransmitter acetylcholine is released, which is important for memory processes that happen during sleep.
It's not that the heart is special — researchers used heart rate variability to measure ANS activity in general. Going forward, they could analyze nappers' cognitive performance with respect to another involuntary process, like respiration, explained Mednick. They could also manipulate the vagus nerve through electrical stimulation or use drugs that alter ANS activity, like beta blockers (used for blood pressure and stage fright).
Of course, Mednick's group isn't the first to focus on the way the body informs feelings and thoughts. The microbiome has been kind of a big deal for the past few years. What's happening inside the gut, we've learned, can change mood and social behavior.
"I think we've seen dominance of the theory that it’s kind of all about the brain — mind over matter," said Mednick. "And I think a study like this might put a little chink in that armor, and make us say, well, maybe there are signals coming in from non-brain areas that make the brain realize [sleeping] is a safe time to do memory consolidation."
Their current findings don't give them license to say there's a causal relationship between sleep-dependent memory and ANS activity. It's possible that heart rate variability, or even vagus nerve activity, is just a by-product of the real, unknown cause. Rednick says it would make sense for the brain not to care about what the body's doing. But the study suggests the brain really does in this case, which is what Mednick finds so surprising. She and her team just need to figure out why.