Two studies conducted at the University of Michigan show that sleep is essential for memory because it allows certain neurons to reactivate and reproduce experiences. Sleep deprivation disrupts this process, which leads to impaired memory retention. Even catch-up sleep does not fully restore memory activation, underscoring the critical role of sleep in memory consolidation.

Imagine you’re a student, it’s graduation week, and you’re preparing for a big exam: Do you study all night or take some rest? As you know, for many people staring blankly at the exam, lack of sleep can make retaining information extremely difficult.

Two new studies from the University of Michigan explore why this happens and what happens in the brain to help or harm memory formation during sleep and sleep deprivation.

Specific neurons can be tuned to specific stimuli. For example, mice in a maze have neurons that fire when the animal reaches certain places in the maze. These neurons, called place neurons, are also active in humans and help people navigate their environments.

So what happens during sleep?

“If this neuron is responding during sleep, what can you tell from that?” said Kamran Diba, PhD, assistant professor of anesthesiology at the UM School of Medicine.

Research summarized in the journal Nature The research, led by Diba and former graduate student Kurosh Maboudi, Ph.D., looks at neurons in the hippocampus, a seahorse-shaped structure located deep in the brain and involved in memory formation, and discovered a way to visualize the coherence of associated neural patterns. along with the location where the animal sleeps.

The role of sharp wave vibrations

For hours at rest and during sleep, a type of electrical activity called a spike pulse occurs from the hippocampus every few seconds.

Researchers were intrigued by how synchronized the wavelets were and how far they likely traveled to spread information from one part of the brain to another. These activations are thought to allow neurons to form and update memories, including those of place.

For the study, the team measured the mouse’s brain activity during sleep after the mouse completed a new maze. Using a type of statistical inference called Bayesian learning, they were able to track for the first time which neurons would respond to which locations in the maze.

“Let’s say a neuron prefers a particular corner of the maze. We can see that the neuron is activated along with other neurons that show similar benefits during sleep. But sometimes neurons associated with other areas can be activated jointly with this cell. Then when we put it back in the maze, we can see which cells fire during sleep.” “We saw that the location preferences of the neurons changed depending on the location.” said.

The method allows them to visualize the plasticity, or representative drift, of neurons in real time. This also provides further support for the long-standing theory that the reactivation of neurons during sleep is part of why sleep is important for memories. Given the importance of sleep, Diba’s team wanted to look at what happens in the brain in the context of sleep deprivation.

Effect of sleep deprivation

In a second study published again NatureA team led by Diba and former graduate student Bapun Giri, Ph.D., compared the extent of neuronal reactivation in which neurons firing during a maze trial spontaneously refired at rest. and the order in which they are reactivated during sleep and during sleep loss (measured as playback).

They found that activation patterns of neurons involved in reactivating and reproducing the maze experience were higher during sleep than during sleep deprivation. Sleep deprivation corresponded to a similar or higher frequency of sharp wave vibrations, but the wave amplitude and power were smaller.

“But in almost half of the cases, the reactivation of the maze experience during sharp waves was completely suppressed during sleep deprivation,” Diba said. Said.

He added that when sleep-deprived mice were able to catch up on sleep, reactivation improved slightly but never matched that of normally sleeping mice. Additionally, reproduction was similarly impaired but was not restored when lost sleep was restored.

Because reactivation and retrieval are important for memory, the results demonstrate the detrimental effects of sleep deprivation on memory. Diba’s team hopes to continue investigating the nature of memory processing during sleep, why they need to be reactivated, and the effects of sleep on memory.