Sleep is essential for memory formation

Imagine you’re a student preparing for a big exam: do you pull an all-nighter or get some rest? As many students know, lack of sleep makes retaining information difficult. 

Two new studies led by the University of Michigan (U-M) have recently revealed why this happens, what occurs in the brain during sleep and sleep deprivation, and how these processes impact memory formation.

Neurons involved in memory formation 

Specific neurons can be tuned to particular stimuli. For example, in a maze, rats have neurons that activate when they reach specific spots. These neurons help with navigation and are also active in humans. But what occurs during sleep?

“If that neuron is responding during sleep, what can you infer from that?” said Kamran Diba, an associate professor of anesthesiology at U-M Medical School, and senior author of both studies.

Diba and his team examined neurons in the hippocampus, a brain structure involved in memory formation, and visualized neuronal patterns associated with a location while an animal sleeps.

Forming and updating memories

Sharp-wave ripples, a type of electrical activity, emanate from the hippocampus every few seconds during restful states and sleep. These ripples are thought to help neurons form and update memories, including spatial ones.

For their study, the researchers measured a rat’s brain activity during sleep after it completed a new maze. Using Bayesian learning, they tracked which neurons responded to which maze locations for the first time.

Reactivation of neurons during sleep 

“Let’s say a neuron prefers a certain corner of the maze. We might see that neuron activate with others that show a similar preference during sleep. But sometimes neurons associated with other areas might co-activate with that cell,” noted Diba.

“We then saw that when we put it back on the maze, the location preferences of neurons changed depending on which cells they fired with during sleep.”

This method allows visualization of neuronal plasticity or representational drift in real time and supports the theory that reactivation of neurons during sleep is crucial for memory.

Lack of sleep and memory formation 

Given sleep’s importance, Diba’s team investigated what happens in the brain during sleep deprivation. 

In the second study, published in the journal Nature and led by Diba and former graduate student Bapun Giri, they compared neuron reactivation – where place neurons that fired during maze exploration fire again at rest – and their sequence (replay) during sleep versus sleep loss.

They found that neuron reactivation and replay of the maze experience were higher during sleep compared to sleep deprivation. A lack of sleep resulted in similar or higher rates of sharp-wave ripples but with lower amplitude waves and power.

“In almost half the cases, however, reactivation of the maze experience during sharp-wave ripples was completely suppressed during sleep deprivation,” noted Diba.

Negative effects of sleep deprivation 

When sleep-deprived rats caught up on sleep, reactivation rebounded slightly but never matched the levels of rats with normal sleep. Replay was also impaired and did not recover with regained sleep.

Since reactivation and replay are vital for memory, these findings highlight the negative effects of sleep deprivation on memory.

Diba’s team aims to further explore memory processing during sleep, the necessity of reactivation, and the impact of sleep pressure on memory.

More about memory formation 

Memory formation involves a complex interplay of neural processes that encode, store, and retrieve information. 

Encoding

This process begins with encoding, where sensory input is transformed into a neural code that the brain can use. 

During encoding, attention and perception play critical roles in determining which information is processed further. 

Storage

Once encoded, the information moves to storage, where it is maintained over time. This can occur in short-term memory, which holds information temporarily, or long-term memory, which can store vast amounts of information for extended periods. 

Consolidation is an essential part of this storage process, involving the stabilization of memories, often during sleep.

Retrieval 

The final stage is retrieval, where stored information is accessed and brought back into conscious awareness. 

Retrieval can be influenced by various factors, including the context in which the memory was formed and cues that trigger recall. 

Brain structures 

The brain structures most involved in memory formation include the hippocampus, which is crucial for consolidating new memories, and the cerebral cortex, where long-term memories are stored. 

Additionally, the amygdala plays a role in emotional memories, highlighting the interconnectedness of cognitive and emotional processes in memory formation.

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