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The link between sleep, memory, and learning has been puzzling scientists for decades. It is clear that the memory forming process is closely related to sleep, and you wouldn’t be able to learn anything without it. But the mechanism behind how it all works is still a great mystery.

A new study from Hokkaido University in Sapporo, Japan, indicates that REM sleep is not only essential for learning, but it could be responsible for our ability to actively forget. It appears that forgetting things isn’t a random process at all, and neurons control it via certain hormones.

We are bombarded with millions of stimuli and new information every day. It would be impossible to remember all of this, which is why your brain must learn to recognize what’s essential and what’s not. Unimportant stuff gets discarded, and our brains don’t get overloaded. And it all happens during REM sleep.

Rapid eye movement or REM sleep is a sleep stage when most dreams occur, especially vivid ones. Scientists have long hypothesized that dreaming is important for memory by sending signals through specific pathways and strengthening neural circuits. But now, this new study on mice indicates that REM sleep could be the time when our brain decides what is not important, and activates mechanisms to forget that information actively.

Some scientists like Francis Crick – the co-discoverer of double-helix DNA structure, have hypothesized about the role of sleep, especially REM phase, in our ability to get rid of the excess information. But before this study, we had no idea about the mechanism of how the brain prunes certain synapses involved in learning.

A team of Japanese and US researchers was particularly interested in neurons producing melanin-concentrating hormone (MCH), which is known to play a role in the control of appetite and sleep. It turns out that the majority of MCH neurons (52.8%) in the hypothalamus are active during REM sleep. Around 35% fire during waking hours, while 12% are active during both states. And it seems that these MCH cells send inhibitory signals to specific memory centers like the hippocampus. 

To test the effects of MCH cell activity, researchers used different genetic tools to turn them on or off and see what kind of impact it would have on memory. They were particularly interested in the retention of new information before it had the time to be incorporated into long-term storage. 

And the results surprised them, as turning on MCH neurons had a negative impact on memory retention, and mice performed worse on memory tests. On the other hand, when MCH neurons were shot down, memory test results improved. That was only effective when managing MCH neurons during REM sleep. Turning them on and off when awake or during other sleep stages had no impact on memory retention. One of the authors, Dr. Kilduff, explained this by hinting that the activation of MCH cells could be to prevent the content of dreams from storing into long term memory. 

Researchers plan to investigate how these new findings could help us learn more about memory-related conditions such as Alzheimer’s, as well as sleep disorders. One thing is sure; sleep remains one of the essentials for a good memory!

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