Recent research suggests that inducing certain sleep-like brain wave patterns in awake mice may provide restorative effects to the brain that are normally only obtained by actually falling asleep. The findings show that the physical need for sleep and the memory-enhancing effects of adequate sleep can be replicated without animals losing consciousness. This study was recently published in the journal natural neuroscience.
Sleep is a biological necessity for all mammals. It works to reset the brain and body after being awake for a long time. When animals are awake, they learn, move, and experience new things in their environment. This arousal activity causes the microscopic connections between brain cells known as synapses to grow stronger and more numerous.
If synapses continue to grow stronger day by day without being reset, the brain becomes physically overloaded, uses up too much energy, and loses its ability to process new information. Deep sleep provides evidence of a massive resetting process throughout the brain. During non-rapid eye movement sleep, which accounts for approximately 80% of total sleep in adults, the junctions between memory-making neurons are assessed.
During this sleep stage, the brain protects important connections for long-term storage, removes less-needed connections, and makes space for new ones. The brain also experiences highly synchronized electrical activity. Millions of neurons fire electrical signals at once, creating what scientists call “on” periods. Immediately after this burst, the cells become collectively silent, known as the “off” period.
This rhythmic back-and-forth switching produces slow brain waves that can be recorded by the sensor. Scientists track this slow wave activity to measure how much sleep animals need. The longer an animal stays awake, the more intense its slow wave activity will be after it finally falls asleep. As the animal rests for several hours, this activity gradually decreases, indicating that its biological need for sleep has been satisfied.
A research team at the University of Wisconsin-Madison, including Cort Driessen, Fabio Squarcio, Giulio Tononi, and Chiara Cirelli, wanted to test these specific questions about brain waves. Researchers have previously shown that both rats and humans can exhibit sporadic, focal slow-wave brain activity during wakefulness when sleep deprived. Although short immersions in sleep-like activities may not be enough to have an effect, the researchers reasoned that doing this activity for longer and more systematically may allow parts of the brain to rest while the animals continue to be active.
“What we’re essentially doing is forcing sleep in a localized region of the brain,” said Chiara Cirelli, a professor of psychiatry at the University of Wisconsin-Madison. “While that part consolidates memories and restores learning abilities, other parts remain conscious, alert, and connected to the environment.” “Dolphins do something similar, sleeping using only one hemisphere of their brain at a time.”
To test this idea, the researchers used a technique called optogenetics. In this method, specific brain cells are genetically engineered so that they can be controlled with a flash of light. Researchers implanted a small light-emitting device and an electrical recording sensor into the brains of adult mice. These implants were placed on both the left and right sides of the brain.
This design allowed the team to manipulate the neural network on one side while using the opposite, untouched side as a natural control for comparison. In the first experiment, the researchers used 19 genetically modified mice. They kept the animals asleep for 5 hours by continuously introducing new objects to the cage.
During the last 30 minutes of this sleep deprivation period, the scientists used light pulses to force neurons on one side of the brain into rhythmic on-and-off periods. They adjusted the light flashes to mimic the precise timing and duration of natural deep sleep waves. During this entire process, the mice were awake and behaving normally, moving around the cage without interruption.
After 30 minutes of light stimulation, the sleep deprivation was resolved and the mice were able to fall asleep naturally. The researchers carefully monitored brain activity during the first hour of this restorative sleep. The side of the brain that was given the artificial on and off periods had significantly reduced slow wave activity compared to the untreated side. Additionally, neurons on the treated side fired much less synchronously.
In sleep science, less synchronization is evidence that the biological pressure for sleep is successfully reduced in that particular area. The authors then asked whether simply reducing overall brain activity without rhythm could have the same effect. Some scientists have suggested that a general reduction in neuronal firing may be a necessary mechanism for recovering from cellular fatigue caused by staying awake.
The researchers conducted a second experiment using seven different genetically modified mice. Instead of creating a rhythmic on-and-off pattern, the scientists used a continuous beam of light to quiet the brain cells, significantly reducing their overall firing rate. When these mice were put to sleep, both sides of the brain showed an equally high need for rest. This finding suggests that the restorative function of sleep requires specific rhythms in which neurons are turned on and off, rather than a general decrease in brain activity.
Next, the research team looked at the physical connections between brain cells. They analyzed molecular markers of synaptic strength in 24 mice. These mice were divided into three groups of eight, including a control group, based on the specific genetic modification. After waking up the mice and applying rhythmic light stimulation to one side of the brain, the researchers immediately harvested brain tissue without putting the animals to sleep.
The researchers measured levels of certain proteins in brain tissue that help transmit signals between neurons. They found that there were significantly fewer of these receptors on the side of the synapse that received light stimulation. This decrease reflects the natural weakening of cellular connections that occurs during normal deep sleep. This particular weakening process tends to prevent the brain networks from being overloaded with information.
Finally, the researchers tested whether this artificial brain rhythm could rescue memories after a period of sleep deprivation. They used a behavioral test of tactile memory, an ability that is highly dependent on rest. The research team used 30 mice in a memory test about floor texture. On the first day, the mice explored an enclosed room with two identical floor textures for 15 minutes.
The animals were then divided into three test groups. Nine mice were returned to their cages and allowed to sleep normally. Thirteen mice were kept awake for 1 hour. Eight mice were kept awake for one hour, during which they received artificial on-off brain stimulation.
The next day, mice were returned to the testing chamber. This time, the room has one familiar floor texture and one completely new texture. Mice naturally like to explore new environments, so well-rested mice will spend more time exploring new surfaces.
The normally sleeping mice recognized the old floor and spent most of their time exploring the new floor. Mice that were just awake could not recognize the familiar floor and spent the same amount of time on both sides. But mice that received rhythmic light stimulation while awake performed just as well as well-rested mice.
These findings are very informative, but require proper contextualization to avoid widespread misunderstanding. Casual readers may misinterpret this study to mean that humans or animals can completely replace a full night’s sleep with localized brain stimulation. The authors note that complete disconnection from the environment, as occurs during natural sleep, is likely still necessary for the brain to process memories on a large system-wide scale. The local stimulation in this study affected only specific target regions of the sensory and motor cortices, rather than the entire brain.
Another limitation is that the method used in this study is highly invasive. Optogenetics involves genetically modifying brain cells and surgically implanting hardware into the skull. For this reason, this exact technique cannot be tested on human subjects. The researchers also noted that artificial brain waves can exhibit opposite electrical polarity compared to naturally occurring sleep waves, depending on the specific cell type targeted.
Future research could focus on how these local periods of rest affect the overall health of the brain over longer periods of time. Cirelli aims to study whether similar effects can be replicated in humans using less invasive techniques such as transcranial stimulation. Understanding exactly how these on and off periods work could ultimately lead to new treatments for severe sleep disorders and age-related memory problems.
“This study further deciphers why we sleep and how we learn, and brings us one step closer to understanding how to better prevent and treat cognitive decline,” said Amy Bunney Adams, acting director of the National Institute of Neurological Disorders and Stroke, which funded the study.
The study, “Induction of cortical on/off periods in awake mice satisfies sleep function,” was authored by Kort Driessen, Fabio Squarcio, Giulio Tononi, and Chiara Cirelli.
