Sleep resets neurons for new memories the next day, study finds

While everyone knows that a good night’s sleep restores energy, a new study from Cornell University reveals that it resets another vital function: memory.

Learning or experiencing new things activates neurons in the hippocampus, a region of the brain that is essential for memory. Later, while we sleep, these same neurons repeat the same pattern of activity, allowing the brain to consolidate these memories, which are then stored in a large area called the cortex. But how can we continue to learn new things throughout our lives without exhausting all our neurons?

A study titled “A hippocampal circuit mechanism for balancing memory reactivation during sleep,” published in Sciencefound that at certain times during deep sleep, parts of the hippocampus become silent, allowing these neurons to reset.

“This mechanism could allow the brain to reuse the same resources, the same neurons, for new learning the next day,” said Azahara Oliva, assistant professor of neurobiology and behavior and corresponding author of the study.

The hippocampus is divided into three regions: CA1, CA2, and CA3. CA1 and CA3 are involved in encoding memories related to time and space and are well studied. Less is known about CA2, which the current study suggests generates this silencing and resetting of the hippocampus during sleep.

The researchers implanted electrodes into the hippocampus of mice, allowing them to record neural activity during learning and sleep. They were able to observe that, during sleep, neurons in the CA1 and CA3 areas reproduce the same neural patterns that develop during daytime learning.

But the researchers wanted to know how the brain continues to learn every day without overloading or running out of neurons.

“We realized that there are other hippocampal states that occur during sleep, where everything is silent,” Oliva said. “The CA1 and CA3 regions that were very active suddenly became silent. This is a memory reset, and this state is generated by the medial region, CA2.”

Cells called pyramidal neurons are considered active neurons that play a functional role, such as learning. Another type of cell, called interneurons, has different subtypes. Researchers have found that the brain has parallel circuits regulated by these two types of interneurons: one regulates memory, the other helps reset memories.

The researchers believe they now have tools to boost memory, by tapping into memory consolidation mechanisms, which could be applied when memory function is failing, such as in Alzheimer’s disease. Importantly, they also have evidence to explore ways to erase negative or traumatic memories, which could then help treat conditions such as post-traumatic stress disorder.

This result helps explain why all animals need sleep, not only to fix memories, but also to reset the brain and keep it active during waking hours. “We demonstrate that memory is a dynamic process,” Oliva said.