Forgetfulness might feel like an irksome flaw of the mind, but a loss of memory is a necessary part of a well-functioning brain. As shown by a new study, the brain might have a specialized set of neurons that work to actively make us forget, hoping to weed out the unnecessary information we collect along our day.

Reporting in the journal Science, researchers discovered that people forget during a specific phase of sleep called rapid eye movement (or REM) via a specific set of neurons found deep inside the brain.  

The cells are known as melanin-concentrating hormone neurons, or MCH for short. These brain cells were previously known for making an appetite-stimulating hormone, but they are now believed to also control the active destruction of memories during REM sleep, the unique phase of sleep in mammals and birds that’s associated with vivid dreams.

By looking at the brains of mice, the team discovered that a majority (over 52 percent) of MCH cells fired when mice underwent REM sleep, compared with 35 percent when the mice were awake. Paired with this, they showed that the MCH cells sent inhibitory messages to the hippocampus, the brain’s memory headquarters.

Using some genetic tweaking, they then created mice that could either have their MCH neurons turned on or off. They discovered that the mice with activated MCH neurons actually had impaired memory during a series of memory tests. To the scientists’ surprise, they found that activating the cells reduced the time mice spent sniffing around new objects compared to familiar ones, while turning the MCH cells off produced the opposite effect.

“Ever wonder why we forget many of our dreams?” Thomas Kilduff, PhD, director of the Center for Neuroscience at SRI International, said in a statement. “Our results suggest that the firing of a particular group of neurons during REM sleep controls whether the brain remembers new information after a good night’s sleep,” he added.

“These results suggest that MCH neurons help the brain actively forget new, possibly, unimportant information,” added Dr Kilduff. “Since dreams are thought to primarily occur during REM sleep, the sleep stage when the MCH cells turn on, activation of these cells may prevent the content of a dream from being stored in the hippocampus – consequently, the dream is quickly forgotten.”

The role of sleep and memory, whether it's forgetting and recollecting memories, is still not fully understood. Nevertheless, it’s hoped that this work could pave the way towards research that deepens our understanding of memory-related conditions, such as Alzheimer's disease.