Rapid eye movement (REM) sleep is a phase of sleep when dreams are created. Traditionally, REM has been thought of as a sleep stage important for memory consolidation, or the long-term storage of memories. Now, a team of Japanese and U.S. researchers show that (in their mouse model) it may also be a time when the brain forgets.
“Ever wonder why we forget many of our dreams?” said Dr. Thomas Kilduff, director of the Center for Neuroscience at SRI International in Menlo Park, CA, 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 fascinating – it first occurs about 90 minutes after falling asleep and is characterized by darting eyes, paralyzed limbs, and dreaming. During REM, our brains are almost as active as when we are awake.
Learn about the stages of sleep below:
It has long been known that sleep is important for new memory storage. But scientists have wondered if sleep – in particular, REM – may also be important for erasing or forgetting excess information. Recent studies have demonstrated that during REM sleep the brain prunes or trims neural connections between cells involved in learning.
"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 Dr. Janet He, program director at the National Institute of Neurological Disorders and Stroke (NINDS). "This study provides the most direct evidence that REM sleep may play a role in how the brain decides which memories to store."
In the study, Dr. Kilduff's lab and that of his collaborator, Dr. Akihiro Yamanaka, at Nagoya University in Japan examined specialized neurons in the hypothalamus, a brain region important for many of the body’s regulatory processes (i.e., hunger, sex, hormones). These neurons produce melanin concentrating hormone (MCH), a chemical needed for sleep and appetite. Researchers found that a majority (52.8%) of the MCH cells fired when mice experienced REM sleep, while only 35% fired when mice were awake. About 12% fired at both times.
Researchers then used genetic tools to turn the MCH neurons on and off during memory tests. Surprisingly, they found that “turning on” the neurons worsened memory retention, whereas “turning off” the cells improved memory. Additional experiments showed an exclusive role for MCH neurons in REM sleep – mice performed better on memory tests when the cells were turned off during REM, but not when they were awake or in another stage of sleep.
"These results suggest that MCH neurons help the brain actively forget new, possibly, unimportant information," said 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."