REM sleep may help the brain forget

Research in mice pinpointed a type of neuron that may help the brain forget things during sleep. Francescoch / iStock / Getty Images Plus

Most dreaming happens during rapid eye movement (REM) sleep. During this stage of sleep, the eyes dart around, and brain waves look similar to those when awake. Muscles are inactive, but heart rate and breathing quicken. This unique sleep stage first happens about 90 minutes after falling asleep. A single night’s sleep can have several sleep cycles that include REM sleep.

Researchers have long suspected that REM sleep is vital to storing memories. Sleep—including REM sleep—may be a time when the brain actively forgets excess information to prevent overload. This idea is supported by recent studies in mice showing that, during sleep, the brain trims away connections between neurons involved in certain types of learning. 

A research team led by Dr. Thomas Kilduff of SRI International partnered with two Japanese teams to explore the brain circuits involved in memory storage while dreaming. Their study was funded in part by NIH’s National Institute of Neurological Disorders and Stroke (NINDS). Findings were published in Science on September 20, 2019.

The researchers used a special staining process to identify neurons that extended into a brain region critical to memory, the hippocampus. They found a set of neurons from the hypothalamus—a small region at the base of the brain. These neurons, which produce melanin concentrating hormone (MCH), are known to be involved in both sleep and appetite. The team suspected that these MCH neurons might also play a role in memory and learning.

The scientist then used genetic tools to turn individual MCH neurons on and off in mice during memory tasks. This included assessing the mice’s ability to distinguish new objects from familiar ones. Mice will spend more time exploring a new object if they are able to remember the familiar ones.

When MCH neurons were turned on during memory retention—the period after learning something new but before the information is stored into long-term memory—the mice’s memory worsened. They spent less time sniffing new objects, as the familiar ones seemed novel to them as well. Turning the MCH neurons off or eliminating them altogether improved memory. As a result, the mice explored the new objects for longer.  

The researchers also found that the effects were exclusive to REM sleep. Mice performed better on memory tasks when MCH neurons were turned off during REM sleep, but turning off the neurons while the mice were awake or in other sleep states had no effect on memory.

The researchers were also able to measure MCH neuron activity in mouse brains. Most MCH neurons in the hypothalamus (53%) fired when the mice were in REM sleep. Others (about 35%) fired while the mice were awake, and the rest (12%) fired in both states.  

“These results suggest that MCH neurons help the brain actively forget new, possibly, unimportant information,” Kilduff says. “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.”