Although it is known that sleep improves cognitive performance, the underlying neural mechanisms, especially in the context of non-REM sleep, are still largely unexplored. A new study by a team of researchers from Rice University and Houston Methodist’s Center for Neural Systems Restoration and Weill Cornell Medical College, coordinated by Valentin Dragoi of Rice University, has nevertheless uncovered a key mechanism by which sleep improves neural and behavioral performance, potentially changing our basic understanding of how sleep optimizes brain performance.

Published in Science, the study shows how NREM sleep—the lighter sleep experienced during a nap, for example—promotes brain synchronization and improves information encoding, shedding new light on this phase of sleep. The researchers replicated these effects using invasive stimulation, which indicates promising possibilities for future neuromodulation therapies in humans. The implications of this discovery potentially pave the way for innovative treatments for sleep disorders and even methods for enhancing cognitive and behavioral performance.

How Sleep Boosts Performance

The research included an investigation of neuronal activity in several brain areas in the macaque monkey species, while the animals performed a visual discrimination task before and after a 30-minute NREM sleep phase. Using multi-electrode arrays, the researchers recorded the activity of thousands of neurons in three brain areas: primary and middle visual cortex and dorsolateral prefrontal cortex, which are associated with visual processing and executive functions. To confirm that the macaques were in NREM sleep, the researchers used polysomnography to monitor their brain and muscle activity and video analysis to ensure that their eyes were closed and their bodies were relaxed. The results showed that sleep improved the animals’ performance on the visual task by enabling them to distinguish between rotated images more accurately. Importantly, this improvement only occurred in those who had actually fallen asleep – the macaques that were awake but calmly so, without falling asleep, did not show the same improvement in performance.

“During sleep, we observed an increase in low-frequency delta wave activity and synchronized discharge between neurons in different cortical regions,” said first author Dr. Natasha Kharas, a former postdoc in Dr. Dragois’s lab and currently in neurological surgery at Weill Cornell. However, after sleep, neuronal activity became desynchronized relative to sleep, allowing neurons to fire more independently. This shift resulted in improved accuracy of information processing and performance on the visual tasks.

The researchers also simulated the neural effects of sleep by low-frequency electrical stimulation of the visual cortex. They applied 4-Hz stimulation to mimic the delta frequency observed during NREM sleep while the animals were awake. This artificial stimulation reproduced the desynchronization effect observed after sleep and similarly improved the animals’ task performance, suggesting that specific patterns of electrical stimulation could potentially be used to mimic the cognitive benefits of sleep.

Developing Therapeutic Brain Stimulation Techniques to Improve Cognitive Function and Memory

According to Dragoi, co-author of the study, professor of electrical and computer engineering at Rice University, the Rosemary and Daniel J. Harrison III Presidential Distinguished Chair in Neuroprosthetics at Houston Methodist, and professor of neuroscience at Weill Cornell University, this finding is significant because it indicates that some of the restorative and performance-enhancing effects of sleep could be achieved without actual sleep. The ability to reproduce sleep-like neuronal desynchronization during wakefulness opens up new possibilities for improving cognitive and perceptual performance in situations where sleep is not possible – such as for individuals with sleep disorders or under challenging circumstances such as space exploration.

The researchers further investigated their results by building a large neural network model. They found that during sleep, both the excitatory and inhibitory connections in the brain weaken, but asymmetrically, so that the inhibitory connections weaken more than the excitatory ones, leading to an increase in excitation. They have discovered a surprising solution that the brain employs after sleep, whereby the neuronal populations involved in the task decrease their degree of synchronization after sleep, even though they receive synchronizing impulses during sleep itself.

The idea that NREM sleep effectively ‘reboots’ the brain in this way, and that this reset can be artificially mimicked, offers potential for developing therapeutic brain stimulation techniques to enhance cognitive function and memory. “This study not only deepens our mechanistic understanding of the role of sleep for cognitive function, but also breaks new ground by showing that certain patterns of brain stimulation could replace some benefits of sleep, suggesting a future in which we could boost brain function independently of sleep itself,” Dragoi said.