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The brain may actively forget during dream sleep

Rapid eye movement, or REM, sleep is a fascinating period when most of our dreams are made. Now, in a study of mice, a team of Japanese and U.S. researchers show that it may also be a time when the brain actively forgets. Their results suggest that forgetting during sleep may be controlled by neurons found deep inside the brain that were previously known for making an appetite stimulating hormone. The study was funded by the National Institute of Neurological Disorders and Stroke (NINDS), part of the National Institutes of Health.

"Ever wonder why we forget many of our dreams?" said Thomas Kilduff, Ph.D., director of the Center for Neuroscience at SRI International, Menlo Park, California, and a senior author of the study published in Science. "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."

REM is one of several sleep stages the body cycles through every night. It first occurs about 90 minutes after falling asleep and is characterized by darting eyes, raised heart rates, paralyzed limbs, awakened brain waves and dreaming.

For more than a century, scientists have explored the role of sleep in storing memories. While many have shown that sleep helps the brain store new memories, others, including Francis Crick, the co-discoverer of the DNA double helix, have raised the possibility that sleep – in particular REM sleep – may be a time when the brain actively eliminates or forgets excess information. Moreover, recent studies in mice have shown that during sleep – including REM sleep – the brain selectively prunes synaptic connections made between neurons involved in certain types of learning. However, until this study, no one had shown how this might happen.

"Understanding the role of sleep in forgetting may help researchers better understand a wide range of memory-related diseases like post-traumatic stress disorder and Alzheimer's," said Janet He, Ph.D., program director, at NINDS. "This study provides the most direct evidence that REM sleep may play a role in how the brain decides which memories to store."

Izawa et al. REM sleep-active MCH neurons are involved in forgetting hippocampus-dependent memories. Science, September 20, 2019 DOI: 10.1126/science.aax9238

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