For most of my adult life, I have noticed something odd. Some evenings, I can barely remember what I did that day. Ask me what I had for breakfast and you will be met with a blank look. On other days, I can tell you the details of interviews I did, the plans I made, which snacks I packed my daughter for school and how much of each she ate.
The difference between them doesn鈥檛 seem to correlate to how I slept. In fact, only one factor seemed to matter: how much I rested. I have long thought that, on days when I am rushing around, I am just more stressed, which .
But there could be another reason: the role of rest itself.
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We all know that a lack of restorative sleep impairs our cognitive abilities. When we are sleep-deprived, we can struggle to , absorb new information and We may also be aware that resting while awake seems to partially restore our sleep-deprived minds.
Now, neuroscientist and his collaborators at the University of Wisconsin-Madison are revealing how certain housekeeping and cognitive tasks that happen during sleep can also occur, to some extent, while we are awake.
This is 鈥渢he last piece of the puzzle鈥 that may answer why wakeful rest helps to fulfil some of the functions of sleep, says neuroscientist at the Paris Brain Institute.
Local sleep
For decades, sleep was understood to be a global state that affects the whole brain at once. In deep sleep, the brain reverberates with slow brainwaves 鈥 in which neurons slowly cycle in sync between firing and silence 鈥 and from the surrounding environment.
But in recent years, research has complicated that perspective. In 2011, a team of sleep researchers, including Tononi and his long-time collaborator , also at the University of Wisconsin-Madison, published a paper describing an odd phenomenon. When rats were sleep-deprived yet awake, their brains showed sudden bursts of sleep-like slow waves 鈥 but only in clusters of neurons at a time. The team called it and considered it to be part of a broader phenomenon that exists in various forms across the animal kingdom (see 鈥淪leeping with part of the brain鈥, at the end of this story).
These bouts of local sleep were incredibly brief: a few hundred milliseconds, at most. But they had a noticeable impact. When rats were trained to reach for sugar pellets, if their sleep-deprived minds had slipped into local sleep first. 鈥淚f it happens at the wrong time, in the wrong area, you have impaired performance,鈥 says Cirelli.
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That isn鈥檛 just true of sweet-toothed rats. Humans also show . And we are more likely to experience both when we are , which is why doing something routine but with high stakes 鈥 like driving a car down a highway 鈥 can be dangerous when we are sleep-deprived.
But Tononi, Cirelli and their colleagues鈥 latest study, , indicates there is an upside to local sleep.
In addition to describing local sleep, Tononi and Cirelli are known for coming up with one of the most compelling explanations for why we sleep: the . As we take in information all day long, our brains form and strengthen thousands of neural connections. These connections are energy-hungry, so we can鈥檛 keep them all. Sleep, they posit, is when our brains go to work decluttering what we don鈥檛 need, which helps with storing what we do.
Could local sleep offer the same sort of housekeeping, just on a micro-scale? To find out, they genetically modified brain cells in mice so that a laser light could switch those neurons 鈥渙n鈥 or 鈥渙ff鈥, mimicking the pattern of slow-wave sleep. In one experiment, they put a local neural network to sleep in awake mice that were sleep-deprived.
Afterwards, they found that the part of the mice鈥檚 brains that had 鈥渟noozed鈥 for 30 minutes showed reduced markers of synaptic strength 鈥 a sign that decluttering had already started 鈥 unlike the part that had stayed fully awake. They also noticed that, during global sleep later on, there was less slow-wave activity in the bit of the brain that had napped, presumably because its housekeeping function had already partly been done 鈥 which also indicated that local sleep seemed to have reduced that brain region鈥檚 need for sleep.
These changes in brain activity were also reflected in behaviour. Normally, if a mouse learns a new task and then is sleep-deprived, its later recall is diminished. But when, in a different experiment, the sleep-deprived mice had local sleep induced across multiple networks for an hour after learning something new, they remembered the task just as well as the mice that took an actual nap. 鈥淭his is the first piece of evidence that suggests that local sleep can do the job of sleep, and not badly,鈥 says Tononi.
Both Tononi and Cirelli are quick to point out that local sleep is no replacement for sleep in general. While they pushed the phenomenon to its limits in the laboratory to see what it was capable of 鈥 and what its underlying purpose might be 鈥 the fleeting bursts of local sleep that we experience in the real world aren鈥檛 the same as a night鈥檚 rest. If your brain can get to work on decluttering connections all at once, as in global sleep, that is clearly better and more efficient, says Tononi.
Still, local sleep seems to be the brain鈥檚 attempt at a fallback. 鈥淭he whole framework of local sleep is it is likely to be adaptive,鈥 says Andrillon. 鈥淚t鈥檚 not a bug of the system. It鈥檚 not just a failure to maintain wakefulness.鈥 When our brain hasn鈥檛 been able to get a global sleep, it does the next best thing.
The question is if, and how, local sleep can be encouraged when it might prove beneficial. Perhaps, some day, humans could benefit from the same sort of technology that Tononi and Cirelli used to induce local sleep in mice. In the meantime, most studies on local sleep have found that the phenomenon arises from sleep deprivation. The more tired we are, the more our brains snatch local snoozes to try to 鈥渃atch up鈥 on housekeeping, much as we might spot-clean the kitchen when we don鈥檛 have time to tidy our entire house.
But researchers including Andrillon emphasise that we might slip into local sleep for other reasons, too. 鈥淎n important aspect of this local sleep framework is that it鈥檚 not just time-dependent 鈥 so how much you have slept 鈥 it鈥檚 also use-dependent,鈥 says Andrillon. 鈥淚f you pick one brain region and overuse it, the probability is that you will see these sleep-like events increase.鈥
One way to 鈥渙veruse鈥 part of your brain? Through highly demanding cognitive tasks 鈥 like, say, writing an article on the science of local sleep.
In 2018, in to examine this effect in humans, a team led by , now at the Oasi Research Institute-IRCCS in Troina, Italy, asked well-rested people to learn the layout of an imaginary city, including 16 landmarks. Then they did a navigation task, without breaks, for almost 2 hours. The longer the test wore on, the worse they did. EEG readings indicated that the same parts of the brain that were most active while learning the city layout also showed local sleep during the navigation task 鈥 at the same moment they made errors.
When people are required to focus on a single-minded task requiring vigilance, it can also exhaust specific neural pathways. A by a team at Washington State University鈥檚 Sleep and Performance Research Center explored how these tasks both push those pathways into local sleep and help explain why people鈥檚 performance on the task tends to get worse the longer they do it.
But could there be ways to trigger local sleep that don鈥檛 depend on getting ourselves tired out first? Andrillon thinks there could be. The monotony we experience during 鈥渕indless鈥 tasks 鈥 when we are, say, folding laundry, knitting or sitting with our eyes closed 鈥 means our overall arousal is lower, he says. That doesn鈥檛 actively exhaust neural networks, pushing them into local sleep. But this low-arousal state may create a permissive environment for聽those networks to take a nap.
This remains a hypothesis. To prove that rest induces local sleep, you would have to use high-density electroencephalography (EEG) to compare the brainwaves of non sleep-deprived people at rest with those of people who aren鈥檛 at rest 鈥 which hasn鈥檛 been done. But, if true, it could help explain something else Andrillon and his colleagues have found: , which tends to occur when we are in a low-stimulation environment. 鈥淚t makes sense that, in this situation, where monitoring the outside is not that interesting, your brain could use these opportunities to do a bit of housekeeping,鈥 says Andrillon.
In his view, we should embrace mind-wandering, as it may well nudge our neurons to take these much-needed 鈥渘aps鈥. However, he says it is unclear whether local sleep is behind the cognitive benefits of entering a state of quiet relaxation with minimal sensory stimulation 鈥 known as wakeful rest.
Ultimately, whether it leads to local sleep or not, wakeful rest is no substitute for global sleep. Taking a break between tasks won鈥檛 make up for getting only 4 hours of sleep. 鈥淭o dissipate sleep need, you need to sleep,鈥 says neuroscientist at Furman University in South Carolina.
The right kind of rest
Still, in some realms, some of the time, wakeful rest may provide similar benefits 鈥 as Wamsley has discovered.
For years, her research focused on how sleep affects memory consolidation. But over time, she noticed something. Across more than a century of research on how sleep affects memory, studies tended to share the same flaw. They would compare people who had slept with those who had done something else 鈥 like watching a movie or continuing with their day. Perhaps it wasn鈥檛 that sleep was benefiting memory consolidation, Wamsley realised. Perhaps it was the absence of other tasks.
To find out, she聽聽in which participants were first given a learning task that involved learning the meaning of Icelandic words and memorising long numbers. The people were then divided into three groups: one that napped, one that played a computer game and one that rested, eyes closed. (Her team used EEG to make sure that those who rested weren鈥檛 actually sleeping.) Then Warmsley and her team tested how well the participants could remember the words and numbers.
Wamsley thought that the napping group would perform better than the others. But, surprisingly, the resting group did just as well. 鈥淩esting with your eyes closed and napping both benefited memory compared to the control condition 鈥 and equally so,鈥 she says.
A lot of this might be because of memory reactivation, she says. During sleep, our brains reactivate the same cellular firing patterns that occurred during the day, something that helps us organise and store our memories. , whether that is zoning out or sitting and closing our eyes.
When we rest, we also aren鈥檛 taking in new information that could interfere with what we have just learned, for example by using up brain resources.
Few studies have compared sleep and rest head-to-head in the same way as Wamsley鈥檚 memory experiment. But some other research has similarly found that, compared with continuing on to other tasks, taking a break may improve our cognitive performance, even for days afterwards.
In one study, for example, participants listened to a story, then spent 10 minutes either sitting and resting quietly or playing a spot-the-difference game. The participants who rested 鈥 and a week later, too. Other experiments have found that sitting quietly for at least 10 minutes after a learning task can we often forget, we need to accurately navigate, and even come up with , in a 鈥淓ureka!鈥 moment. Rather than 鈥渟leeping on it鈥, resting on it may be enough.
has found that wakeful rest has an effect. But one analysis from January found that, across 51 studies, . The benefits were strongest for older adults and people with memory impairments like amnesia.
In our always-on culture, it has never been harder to find the time for pauses between tasks. But above all, don鈥檛 pack your day, says Andrillon. 鈥淒on鈥檛 overdo it, because there are things that escape us that our brains are doing. And these things are pretty important.鈥
On that note, it is time for me to close my laptop, sit quietly for 10 minutes and, just maybe, let my neurons take a micro-nap.
Half asleep
Recent research has shown how the phenomenon of 鈥渓ocal sleep鈥, in which small clusters of neurons briefly snooze, exists in animals and seems to have cognitive benefits for humans (see main story). But some other species have evolved a related way to gain the benefits of sleep without fully switching off: unihemispheric sleep, where half the brain goes to sleep at a time. In fact, this can be core to their survival.
Take the northern fur seal, which migrates thousands of kilometres each winter, then stays in the water for about 10 months before returning to land. When on land, this seal sleeps much like a human, complete with a global sleep cycle across its entire brain. But , it stays partly alert because predators pose a greater threat.
One eye remains open, one flipper paddles for balance, one set of whiskers is active, monitoring its surroundings 鈥 and one side of the brain remains in an active, 鈥渁wake鈥 state, while the other one sleeps. Unihemispheric sleep has also been found in other aquatic mammals, including , and .
Similar behaviour is seen in various . Great frigatebirds stay aloft for weeks at a time as they migrate, and can . But this doesn鈥檛 just happen during migration: when mallards are flanked by other birds, for example, they show brainwaves indicating global sleep, but when they are at the group鈥檚 edge, .
