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Sleep on It: You May Forget It

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New research has concluded that while you sleep, your brain decides which memories are worth keeping and which ones to forget.  It’s all about a specific type of neuron that is activated during deep REM (round eye movement) sleep.

The neurons are called melanin-concentrating hormone-producing hormones (otherwise known as MCH).  MCH neurons are in the hypothalamus of the brain which are relatively active while we are awake, completely inactive during the stage of non-REM sleep, and extremely active during REM sleep.  A team of Japanese scientists have determined how and why these MCH neurons function as they do.

Emotional attachment to memories comes from the amygdala portion of the brain.  The MCH neurons help your brain to forget unnecessary memories while you are in the REM stage of sleep.  The physiological role of sleep has been theorized to be important for removing toxins from the brain, and this new research offers another reason for sleep:  clearing up space within the brain as we sleep.

MCH neurons can activate all memories to be erased during REM sleep, but the memories with emotional input (the so-called “important” memories) are left alone because the connection of emotional attachment from the other region of the brain (the amygdala) marks them as important to the brain.

Far from being completely understood, this research may be the forerunner to some interesting developments.  The scientists were able to turn these MCH neurons on and off by exposing the brain to light or by introduction of a pharmaceutical substance.  In mice studied whose MCH neurons were turned off while they were in REM sleep, their ability to score higher on memory tests increased; and when the MCH neurons were activated, they scored worse on memory tests.

This research gives more credence to the saying “sleep on it,” because if it is unimportant to you, you might just forget it.


  1. Izawa, Shuntaro, et al. “REM Sleep–Active MCH Neurons Are Involved in Forgetting Hippocampus-Dependent Memories.” Science, vol. 365, no. 6459, 20 Sept. 2019, pp. 1308–1313., doi:10.1126/science.aax9238.


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