Why do we spend a third of our lives sleeping?

Studies have found that insomnia can lead to a confusion of electrical activity in neurons, which hinders the writing of memories.

Written by Ian Sample

Translated by Zhang Xue

Reviewer Hu Jiaxian

Scientists have found that when the brain is not adequately rested, the neurons in it seem to over-connect, and the electrical activity of the brain is also somewhat chaotic, so that newly occurring memories cannot be properly processed. Image source: www.bridgemanimages.com

Why do we spend a third of our lives sleeping? There is still a lack of scientific explanation for the problem, but research on the damage to the brain caused by insomnia at night has shed some light on us and offers exciting clues about potential treatments for depression.

In a new study published in August, researchers found for the first time that sleep can reset the stable neuronal connections that the brain makes when awake. This process is important for the brain's memory and learning so that we can adapt to the world around us.

Scientists have found that just one night of sleep deprivation is enough to block the brain's natural reset mechanism. Once deprived of rest time, neurons in the brain appear to be overconnected, and electrical activity in the brain is disrupted, so that newly generated memories cannot be properly processed.

Christoph Nissen, a psychiatrist at the University of Freiburg in Germany, who led the study, was thrilled by the potential function of the study to help people with mental illness. One of the root treatments for major depression is sleep deprivation, which is not keeping patients from sleeping, and Neeson believes that this therapy works by altering the connection of a patient's brain. The new study provides a deeper understanding of the phenomenon and has the potential to produce more practical treatments.

"Why do humans sleep? This is a fundamental problem. Why do we spend so much of our lives on this brain state? This study tells us that sleep is a highly active process in the brain and is not a waste of time. Healthy brain function requires sleep. Neeson said.

The results of this study also support the synaptic homeostasis hypothesis (SHY) hypothesis of sleep, which was proposed in 2003 by scientists at the University of Wisconsin-Madison. Our brains need to receive a lot of information throughout the day, whether it's morning news, weather conditions, or the ingredients needed for after-lunch gossip and tea, and this theory can explain why our brains need to rest after absorbing all kinds of information throughout the day.

The SHY hypothesis holds that when we are awake, as the learning process progresses, the synapses that form the neuronal connections in the brain can be continuously strengthened, eventually filling the brain with various types of information. This process requires a lot of energy, and sleep flattens brain activity, consolidates memories, and prepares for the cycle the next morning.

In the paper, published in the journal Nature Communications, Nissen describes the results of a series of trials involving 11 men and nine women between the ages of 19 and 25, some of whom experienced a good night's sleep and some of whom had not slept the night before. During sleepless nights, participants were allowed to play games, walk or cook food, but caffeine was not allowed. The researchers watched the whole process to make sure they stayed awake.

In the first round of experiments, Nissen used magnetic pulses to activate neurons in the volunteers' brains and trigger twitches in the muscles of his left hand. For volunteers who have not slept, very weak pulses are enough to trigger the movement of the muscles. This suggests that the brain, which lacks sleep, is in a more excited state, and the neurons in the brain are more connected than if they were enjoying a good night's sleep.

Next, Neeson performed another form of brain stimulation to simulate how neurons activate when memories are written. He found that neurons in the sleep deprivation experimental group were more difficult to respond to, which also showed that the writing memory process was impaired by sleep loss.

In summary, the findings suggest that sleep calms down the brain's activity so that memories can be written. In contrast, sleep-deprived brains are disturbed by disrupted electrical activity, making it so difficult to complete memory function that the process is almost blocked. The consequences of sleep deprivation can be visualized by a simple memory test in which exhausted volunteers performed much worse than those who had undergone adequate rest. But how exactly does sleep affect brain connections? Distilling the mechanisms out of them may be far more important than answering the question of "why do we sleep?" itself. Understanding such a mechanism can help many night workers and military personnel who need to overcome sleep deprivation problems, and develop new drugs or countermeasures to help them restore the normal connection of the brain. In the study, blood samples from volunteers showed that sleep deprivation lowered levels of brain-derived neurotrophic factor (BDNF), a molecule that regulates synaptic connections in the brain.

But Neeson is more interested in the treatment of depression, and this study has helped scientists better understand sleep deprivation treatments and their efficacy for depression. "If you keep people with major depression from sleeping all night, about 60 percent of patients will show substantial improvements in mood, motivation, and cognitive function. We believe this is because this therapy can transition these patients to a more favorable state. He said.

Although the efficacy of sleep deprivation therapy is amazing, it is not widely used because many patients relapse during subsequent nighttime sleep. But Neeson thinks that's not the point. "It proves that it's possible to transition a person's different mood states in just a few hours." The idea is to use sleep and sleep deprivation to understand the brain and develop novel treatments. If antidepressants or psychotherapy are used, it can take weeks or months to see results. ”

Giulio Tononi, a professor of sleep medicine at the University of Wisconsin-Madison, the first scientist to propose the SHY hypothesis, called the new study "very subtle and powerful" and confirmed the results of experiments that can only be performed in animal trials at the moment.

"One of the main reasons sleep is critical is that it allows the brain to store and consolidate old memories as it learns new things every day," Tononi says, "and learning and memory require synaptic activity, which is very energy-intensive and easy to saturate." During sleep, the brain is able to re-regulate synaptic activity that gradually increases during the day to normal levels. ”

Lars Westlye, a psychologist at the University of Oslo in Norway, praised the study as "brilliant," saying the results could shed light on the biology of sleep, complex brain function, and links between many mental illnesses, such as schizophrenia and depression. Like Neeson, he thinks a clearer understanding of brain connectivity might explain why sleep deprivation therapy is so effective in people with depression, and plans to study the efficacy in these patients.

"These new findings can strongly motivate patient groups to further their research, which not only leads to a better understanding of the root causes of the disease, but also to the development of new treatments." Lars said.

Original link: https://www.theguardian.com/science/2016/aug/23/sleep-resets-brain-connections-crucial-for-memory-and-learning-study-reveals

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