In my last two posts on sleep, I reviewed the long-term health benefits of getting enough sleep and how you can get more if you don’t get enough already. In this final post on sleep, I’ll review tentative evidence about the possibility of going beyond just getting enough sleep, not by getting more sleep, but by getting unusually good sleep. The studies I’ll discuss here are all small and short-term. In other words, they’re probably wrong. But they suggest some fun self-experiments and dangle the promise of becoming smarter, more creative, or more of whatever other quality you want to squeeze out of your brain.
First, some review from Matt Walker’s Why We Sleep. The deepest sleep is called slow-wave sleep because the frequency of your brainwaves slows to 2-4 Hz, roughly 10 times slower than when you’re awake or in REM sleep. Within slow-wave sleep, though, there are periodic bursts of higher frequency brain waves lasting a few seconds each, called sleep spindles. These sleep spindles turn out to be a great way to measure the quality of someone’s sleep.
In one experiment, Walker’s group took a bunch of students and divided them into two groups. At noon, both groups had to memorize names to go with 100 faces. Then, one group took a 90-minute nap while the other group played board games or browsed the internet. Finally, both groups had to memorize a second batch of 100 names. Although the groups had performed similarly well on the first batch of names, the group that took a nap improved on the second batch while the group that stayed awake got worse. The experiment went a step beyond showing that getting sleep improved performance on the test. They also used electrodes to measure the brainwaves of the nappers. It turned out that the number of sleep spindles was correlated with the level of improvement between the first and second tests.
In another version of the experiment, subjects were given a list of words to memorize, one word at a time. After each word, the subjects were told they needed to forget or to remember the prior word. At the end of the day, after half the subjects took a nap and half stayed awake, everyone was asked to recall as many words as possible from the list, whether they had been told to forget the word or remember the word. Nappers had much better recall than the non-nappers for the words they had been told to remember, but not the words they had been told to forget. Walker argues that our sleeping brains are able to sort out which information needs to be stored and which doesn’t. Again, within the napping group, the number of sleep spindles an individual had was positively correlated with their memory after their nap.
Experiments on tests of motor skills showed similar results: the rate at which we learn is positively correlated with the number of sleep spindles between learning sessions. Walker cites many more experiments that didn’t go to the trouble of measuring sleep spindles but found that slow wave sleep (when sleep spindles occur) is the phase of sleep most strongly correlated with retention of newly learned information. It seems like a reasonable guess, then, that if we can get more sleep spindles we’ll get more benefit from a fixed amount of sleep.
Can we actually influence how many sleep spindles we get? This is where we really tip into science fiction. Sleep scientists have been experimenting with transcranial direct current stimulation (tDCS), or electrodes that give very mild electric shocks in rhythm with a sleeping person’s brain waves. The last part will become important in a moment. Walker describes a 2006 German randomized controlled trial of tDCS. Both groups in the trial memorized a list of facts, slept for a night, and were tested on the facts the next day. tDCS “almost doubled the number of facts that individuals were able to recall the following day, relative to those participants who received no stimulation.” Similar experiments using “quiet auditory tones being played over speakers next to the sleeper” achieved similar, though more modest, results, but only if the “tones are precisely synchronized with the individual’s sleeping brainwaves.” Importantly, tones played out of rhythm with the sleeper’s brainwaves actually disrupted sleep.
What does the literature say about tDCS beyond this one experiment? First, this 2004 experiment on 30 healthy young adults found memory benefits using constant current tDCS. The same research group put out a second paper in 2006, apparently the oscillating current paper Walker references in his book. However, this “almost identical” 2013 study of 26 healthy elderly adults found no benefit from tDCS. Conflicting evidence like this calls for a meta-study. Luckily one exists. Unsatisfyingly, this meta-study finds that electrical brain stimulation both enhances and disrupts memory consolidation during sleep. The full text is stuck behind a paywall, so I can’t figure out what they mean by that, but it sounds like brain stimulation for better sleep is still firmly planted in needs-more-study territory.
There are plenty of shady looking companies online that will sell you devices to experiment with tDCS. The currents they use are extremely small, so it seems safeish, but I wouldn’t spend a few hundred dollars on such shaky evidence. The mixed evidence above is based on trials run by people who turn on the stimulation only for brief periods while a subject is asleep and who make sure any oscillation is in rhythm with the subject’s brainwaves. You won’t be able to do that on your own. Another challenge with DIY brain stimulation is that “the relative location of electrodes results in significant differences in where and how much current is delivered to the brain….as little as 1 cm of movement in electrode positions significantly altered the distribution of predicted current flow in the brain.” I for one will be passing on electrical brain stimulation for the time being, but I wouldn’t be too surprised if some day there were a mildly useful consumer device you could put on your head at night to enhance your sleep. In the meantime, if you want to be a competitive sleeper, you’ll have to stick to exercise and melatonin.