Using electrodes to stimulate people's brains with electrical currents while they slept, researchers at a neuroscience lab in Oregon successfully lengthened the amount of time the subjects stayed in the deep sleep stage, which can potentially improve memory and decrease the risk of dementia in older adults.
In a paper published March 8 in Sleep Medicine, a team of neuroscientists at the Brain Electrophysiology Laboratory Company in Oregon tested the effect of electrically stimulating the limbic cortex region of the brain on deep sleep quality in adults. They found that even low-level electrical stimulation through electrodes can significantly increase the duration of deep sleep, contributing to memory formation.
Functions of the human brain deteriorate with age, including memory, which is associated with poor sleep quality in aging. Deep sleep in particular, the deepest stage of non-rapid eye movement sleep and the sleep stage with the slowest brain waves, has an impact on storing memories.
Don Tucker, a neuroscience professor at the University of Oregon, the founder of the BEL Company and a co-author of the paper, explained in an interview with The Academic Times that in deep sleep, the brain generates large slow oscillations, which were previously assumed to be distributed across the brain. But in recent research, scientists at the BEL Company, a laboratory and neuroscience technology company, discovered that the large slow oscillations of human deep sleep are actually generated by specific regions of the brain, in the limbic system.
Memory formation has been thought to be a brain function that takes place when an individual is awake and actively thinking. But Tucker said that in the last 20 years, it's been newly understood that during sleep, the brain replays the memories of the day in order to store them and integrate them with existing memories. And the large slow waves of deep sleep are important to the communication between the brain regions responsible for memory consolidation, such as the limbic system and the neocortex.
During deep sleep, the cerebrospinal fluid from the brain is also flushed out. If adults don't regularly reach deep sleep, it's harder for their brain to do this, and they are more likely to be at risk for various forms of dementia, including Alzheimer's disease.
The discovery that the limbic areas are actually generating the large slow oscillations during deep sleep was very important, because it gives neuroscientists another clue into the dynamics of cortical limbic consolidation and communication, Tucker said. This led him and his colleagues to develop the current study.
"We're excited about these results, and we're moving to try to develop practical sleep therapy. And this paper shows that it should be feasible," Tucker said.
The study involved 13 healthy adults who participated in three all-night sleep sessions. The researchers originally planned for a larger sample with a diverse age range, but the study was halted in March 2020 with just 13 participants due to the COVID-19 pandemic.
In the first night, every participant had their sleep recorded with an electroencephalogram, known as an EEG. For the second and third nights, the participants either received transcranial electrical stimulation, a noninvasive technique that passes an electrical current through the cortex of the brain, or a placebo procedure, in addition to EEG monitoring. The subjects were unaware of which night they received the electrical stimulation and which night was the placebo.
The electrical stimulation was administered in five blocks of five minutes each while the participants were in the deep sleep stage. The electrodes were placed specifically on their frontal brain areas to target the limbic sources of the slow oscillations that occur during deep sleep.
"Knowing these sources, we could then use computational modeling to stimulate these areas efficiently through applying current to the head, thereby synchronizing the slow oscillations," Tucker said.
"We were able to do this with half of the current level, demonstrating the efficiency. We first thought low current would be important to keep from waking people up, but we're no longer worried about that," he continued.
Overall, the participants spent about 63 minutes, or 13.2% of the night, in deep sleep during their stimulation session, which was more time than during their placebo session, when they stayed in the deep sleep stage for about 56 minutes, or 11.5% of the night. Neither time in minutes nor percent time spent in other sleep stages differed significantly between the stimulation and placebo conditions, the authors said in the paper.
Individually, the older participants spent less time in deep sleep than the younger participants. The electrical stimulation in the first deep sleep period of the night also led to greater rapid eye movement sleep later in the night, which the authors suggested should be further explored in future studies.
Tucker said that deep sleep begins to decline in quality at age 30, putting adults at risk for developing memory disorders such as Alzheimer's. Improving sleep quality can help reduce this risk, and his laboratory has been working on this research and perfecting the technology for several years.
Thus far, the scientists have built a prototype of a headband that people would be able to wear at home while they sleep. The headband would be connected with Bluetooth to a bedside nano computer that would score their sleep stages, and then apply the currents to their brain at the right time to synchronize the slow oscillations.
"We're pretty confident it will work. But the question is, does it work long enough to really help people sleep better?" Tucker said, explaining that the researchers know the effects can last through a night. Even when the current is shut off, the brain stays in the rhythm of deep sleep.
"But what about the next night, or the next night? Would you have to wear the headband every night to get a good night's sleep, or maybe, does the brain become more youthful and start sleeping better on its own?" he continued.
It will take years of further research to definitively know if this type of sleep therapy can really make a difference in the quality of memory and sleep, according to Tucker. To get there, he plans to work with older adults who are willing to try this stimulation method for weeks or months at a time to measure if it has an impact in the long run.
The study, "Transcranial Electrical Stimulation Targeting Limbic Cortex Increases the Duration of Human Deep Sleep," published March 8 in the Sleep Medicine journal, was authored by Don Tucker, University of Oregon and the Brain Electrophysiology Laboratory Company, Evan Hathaway, Kyle Morgan, Megan Carson, Roma Shusterman, Mariano Fernandez-Corazza and Phan Luu, the BEL Company.