Source: Medical Daily

Now, a research team from the U.S. and Japan have a potential answer to the widely asked question of why people forget dreams. They found the brain actively forgets when an individual gets into the rapid eye movement (REM) sleep. 

REM is a stage of sleep that happens 90 minutes after shut-eye. It involves paralyzed limbs, raised heart rates, darting eyes and dreaming. 

The new study, published in the journal Science, shows forgetting during sleep occurs because of certain neurons that are also linked to production of an appetite stimulating hormone. 

“Ever wonder why we forget many of our dreams?” Thomas Kilduff, senior study author and director of the Center for Neuroscience at SRI International in California, 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.”

Kilduff and his colleagues analyzed the brain cells that produce melanin concentrating hormone (MCH) in mice. This molecule contributes to sleep and appetite. 

The researchers said 52.8 percent of hypothalamic MCH cells appeared active when animal subjects underwent REM sleep. Nearly 35 percent of the cells fired when the mice were awake. 

The team then conducted another test to see how MCH affects learning and memory. They used genetic tools to turn neurons on and off and provided memory tests. 

Results showed that active MCH cells decreased the animals’ ability to store new information, while turning them off improved memories. 

The MCH also appeared active during REM sleep. Researchers said mice performed better on memory tests when the neurons were turned off during sleep.

“These results suggest that MCH neurons help the brain actively forget new, possibly, unimportant information,” Kilduff said. “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 team plans to conduct a new study to see how sleep affects memory disorders. Understanding how forgetting during sleep may help provide new approach to memory-related diseases, such as post-traumatic stress disorder and Alzheimer’s, according to Janet He, program director at the U.S. National Institute of Neurological Disorders and Stroke (NINDS), which funded the study.