While You're Sound Asleep, Your Brain Removes 'Unnecessary' Memories

The hypothalamus section of your brain is still working hard during your resting hours.
Fabienne Lang

As you lay sound asleep with your head on your pillow, you probably don't imagine that your brain is still madly working away. One of the many actions the brain is busy with is sifting through your daily moments and thoughts. 

Researchers from Japan have discovered neurons in our hypothalamus, whose inhibition during rapid eye movement (REM) sleep is linked to removing 'unnecessary' memories from our brain. This is the part of the brain that has several functions, namely, linking our nervous system to our endocrine system.


Our brains while we sleep

The brain is still a mysterious organ; however, this particular research assists in uncovering and understanding the unknown neural mechanisms that work on our memory regulation while we sleep

While You're Sound Asleep, Your Brain Removes 'Unnecessary' Memories
The activity of melanin-concentrating hormone (MCH)-producing neurons impaired memory during REM sleep. Source: Panoji

As we sleep, our brain works hard at processing all of the information we gathered throughout the day, classifying it and placing it in certain 'boxes' within the brain — consolidating them into memories. 

That said, certain daily experiences are not worth remembering — according to our own brain — and are thus expunged from our memory. 

This is, in fact, an incredibly useful process. Otherwise, we'd have an overflow of information that our brain wouldn't know how to handle. 

While You're Sound Asleep, Your Brain Removes 'Unnecessary' Memories
Brain state-dependent inhibition of melanin-concentrating hormone (MCH)-producing neurons using optogenetics revealed that memory was improved if MCH neurons were inhibited during REM sleep. Source: Akihiro Yamanaka/ RIEM, Nagoya University

Forgetting is essential to our memory regulation, and the process of removing unnecessary memories is called synaptic renormalization. This process can only occur during our sleeping hours. 

Where the hypothalamus comes into play

The team of researchers, led by Shuntaro Izawa from Nagoya University in Japan, investigated the role of a specific set of neurons called the Melanin-concentrating hormone (MCH) in sleeping mice. 

MCH neurons are exclusively found in the hypothalamus and are believed to play a role in the regulation of sleep and wakefulness, including time spent in the REM portion of sleep. 

While You're Sound Asleep, Your Brain Removes 'Unnecessary' Memories
The hypothalamus-pituitary complex. Source: OpenStax College/Wikimedia Commons

The team used a combination of chemogenic and optogenetic techniques to observe the role of MCH neurons in memory regulation in the mice. 

What did the team discover? 

Much to their surprise, Izawa and his colleagues found that inhibition of the neurons increased the memory in the rodents, while MCH neurons impaired memory.

So, the results suggest that our REM sleep neural pathway plays a large role in our active forgetting. The researchers look to the future, in which MCH pathways could be used as a target for memory regulation. 

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