Why Deep Sleep Boosts Our Memory

It has been known for almost 20 years that slow, synchronous waves of excitation during deep sleep support memory formation. It was previously unknown why this is the case. In the journal Nature Communications, a research team from the Charité - Universitätsmedizin Berlin has now provided an explanation. According to them, the slow waves make the cerebral cortex, the seat of long-term memory, particularly receptive to information. The findings could help to optimize treatment approaches that are intended to support memory formation from the outside.

How do lasting memories develop? Experts assume that our brain replays the events of the day while we sleep, moving the information from the seat of short-term memory, the hippocampus, to long-term memory in the cerebral cortex. The so-called "slow waves" are particularly important for this memory formation: slow, synchronous waves of excitation in the cerebral cortex that occur in the deep sleep phase and can be measured using an electroencephalogram (EEG). They are due to the fact that the electrical voltage of many nerve cells simultaneously oscillates up and down once per second.

"We have known for many years that these voltage fluctuations contribute to memory formation," explains Prof. Jörg Geiger, director of the Institute of Neurophysiology at the Charité and head of the study that has now been published. "Because if you artificially enhance slow-wave sleep from the outside, memory performance improves. However, we did not know until now what exactly happens in the brain because the flow of information in the human brain is extremely difficult to research."

Slow waves strengthen synapses

Using particularly rare human brain tissue, he and his team have now succeeded in clarifying the processes that most likely underlie memory formation during deep sleep. According to their findings, the slow waves of excitation influence the strength of the synaptic connections between the nerve cells in the cerebral cortex - and thus their ability to absorb information.

For the study, the research team examined intact tissue from the cerebral cortex of 45 patients who had undergone neurosurgery to treat epilepsy or a brain tumor at the Charité, the Evangelisches Klinikum Bethel or the University Medical Center Hamburg-Eppendorf. The scientists simulated the voltage fluctuations in the tissue that are typical of slow waves in deep sleep and then measured the reaction of the nerve cells. To do this, they used extremely fine glass pipettes, which they docked onto individual nerve cells with nanometer precision. In order to listen to the communication of several neurons in the tissue network, they used ten "pipette sensors" at the same time - a particularly high number for this method known as multipatch technology.

Perfect timing promotes memory formation

The research team found that the synaptic connections between the neurons in the cerebral cortex are maximally strengthened at a very specific point in time during the voltage fluctuations. "The synapses work most efficiently immediately after the voltage has risen from a low level to a high one," explains Franz Xaver Mittermaier, scientist at the Institute of Neurophysiology at the Charité and lead author of the study. "Within this short time window, the cerebral cortex is essentially in a state of heightened readiness. If the brain plays back a memory right now, it is transferred to long-term memory particularly effectively. Slow-wave sleep therefore apparently supports memory formation by making the cerebral cortex particularly receptive for many short periods of time."

It may be possible to use this knowledge to improve memory performance, for example in cases of early forgetfulness in old age. Research groups around the world are working on methods to influence the slow waves of sleep using subtle electrical impulses - transcranial electrical stimulation - or acoustic signals. "At the moment, however, such stimulations are laboriously optimized through trial and error," says Jörg Geiger. "This is where our findings on perfect timing could help. They allow for the first time to develop targeted stimulation methods to support memory."

Reference: Mittermaier FX, Kalbhenn T, Xu R, et al. Membrane potential states gate synaptic consolidation in human neocortical tissue. Nat Commun. 2024. doi: 10.1038/s41467-024-53901-2

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