New study hints at how naps improve performance

For more than a century, we've known that sleep boosts cognitive performance. The big question is: how, exactly?

One popular idea is that toxins which accumulate in the brain while we're awake are cleared away during a good night's sleep. But this theory, known as the glymphatic system hypothesis, can't explain how people can perform better on a taxing test after just a brief nap.

New research, led by Natasha Kharas at Weill Cornell Medical School, provides some compelling insights on why this happens, with data suggesting that it might even be possible to use brain stimulation techniques to mimic the beneficial effects of sleep while awake.

Kharas and her colleagues conducted their experiments on macaques. The team first trained five of the monkeys on a visual task in which they had to decide whether two objects that were briefly flashed on a screen were identical or not. These images were rotated at a variety of angles, by up to 90 degrees, making the task reasonably demanding.

After performing a set of these tasks, the animals either napped for half an hour (in non-REM Stage 1 and Stage 2 sleep) or rested while awake. During this time, the team used electrodes to record the activity of more than 4,000 neurons in three areas of their cortex. Two of these were visual areas (V1 and V4). The third was the dorsolateral prefrontal cortex, a region involved in decision-making that is active during the visual task. After the thirty-minute period, the monkeys were given exactly the same set of tasks to do again.

The team found that only those that had napped did better on these tasks the second time around. Neuronal recordings revealed some hints as to why. During a nap, there was an increase in low frequency delta wave activity and a decrease in the gamma band (which is associated with wakefulness). The team also observed that neurons in the different regions fired in sync. After the napping animals woke up, however, neuronal activity became more out-of-sync than it had been beforehand. None of these changes were seen when a monkey had rested without snoozing.

The team think that the increase in out-of-sync firing after the napping animals woke up allowed neurons to fire more independently, which then boosted their accuracy in processing information, explaining their post-nap improvement on the visual tasks.

As the team notes, these findings suggest that increased delta wave activity during non-REM sleep leads to the beneficial 'desynchronisation effect' on cognitive performance after waking. But in the next stage of their study, they firmed up the evidence for this.  

This time, the macaques always stayed awake between the two blocks of visual tasks. But in some trials, during the in-between rest phase, the team applied an electric current to their brains. This current mimicked the delta frequency observed during the non-REM sleep.

Their results showed that this artificial stimulation caused the same desynchronisation effect that they'd seen in the earlier study — and an improvement on the visual task. "This finding is significant because it suggests that some of the restorative and performance-enhancing effects of sleep might be achieved without the need for actual sleep," commented senior author Valentin Dragoi in a statement released at the time.

Further studies are now needed to explore whether stimulating delta wave activity in people could have the same effect, and to explore what happens to performance if the stimulation lasts for longer than half an hour. While this new work does not mean that the glymphatic system hypothesis is wrong — or, of course, that REM sleep, which brings its own benefits, can be ignored — it could potentially open a new route to helping those who struggle to get enough sleep. In theory, this could mean astronauts on an extended mission, special forces operatives — or just anyone who suffers from insomnia.

Read the paper in full:
Kharas, N., Chelaru, M. I., Eagleman, S., Parajuli, A., & Dragoi, V. (2024). NREM sleep improves behavioral performance by desynchronizing cortical circuits. Science, 386(6724), 892–897. https://doi.org/10.1126/science.adr3339

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