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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’t 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 may affect memory formation.
But there could be another reason: the role of rest itself.
We all know that a lack of restorative sleep impairs our cognitive abilities. When we are sleep-deprived, we can struggle to recall what we have learned, absorb new information and sustain focus. We may also be aware that resting while awake seems to partially restore our sleep-deprived minds.
Now, neuroscientist Giulio Tononi 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 “the last piece of the puzzle” that may answer why wakeful rest helps to fulfil some of the functions of sleep, says neuroscientist Thomas Andrillon 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 – in which neurons slowly cycle in sync between firing and silence – and mostly disconnects 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 Chiara Cirelli, 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 – but only in clusters of neurons at a time. The team called it local sleep and considered it to be part of a broader phenomenon that exists in various forms across the animal kingdom (see “Sleeping with part of the brain”, 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, they were more likely to fall short if their sleep-deprived minds had slipped into local sleep first. “If it happens at the wrong time, in the wrong area, you have impaired performance,” says Cirelli.
That isn’t just true of sweet-toothed rats. Humans also show local sleep states just before experiencing an attentional lapse. And we are more likely to experience both when we are tired, which is why doing something routine but with high stakes – like driving a car down a highway – can be dangerous when we are sleep-deprived.
But Tononi, Cirelli and their colleagues’ latest study, published in June, 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 synaptic homeostasis hypothesis. 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’t keep them all. Sleep, they posit, is when our brains go to work decluttering what we don’t 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 “on” or “off”, 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’s brains that had “snoozed” for 30 minutes showed reduced markers of synaptic strength – a sign that decluttering had already started – 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 – which also indicated that local sleep seemed to have reduced that brain region’s 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. “This is the first piece of evidence that suggests that local sleep can do the job of sleep, and not badly,” 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 – and what its underlying purpose might be – the fleeting bursts of local sleep that we experience in the real world aren’t the same as a night’s rest. If your brain can get to wo
rk on decluttering connections all at once, as in global sleep, that is clearly better and more efficient, says Tononi.
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Jason Ford
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