Shiny, Happy People Need REM Sleep

A new study finds more proof that REM sleep is critical to memory function. Getty

It's challenging for researchers to study how sleep affects neurological function, in part because depriving humans of shuteye is a little unethical. So scientists often turn to animals to gain a clearer understanding of how—and why—a full night of undisturbed rest is essential for things like motor skills, cognitive function, critical thinking and, especially, memory. However, it's not enough to know that sleep quality and quantity impacts these things. In order for sleep specialists to improve their ability to help patients, researchers must identify which patterns of nighttime unrest can cause problems during the day.

A new study published May 13 in Science suggests disruption to rapid eye movement (REM) sleep—the fifth stage in a regular sleep cycle—is linked to poor memory. REM sleep is critical, and is characterized by fluttering eye movements, temporary limb paralysis, rapid heartbeat, lower body temperature and vivid dreams. On average, a human spends approximately two hours each night in REM sleep, and in an eight-hour night of sleep this stage occurs about four to five times. REM sleep is essential because it's tied to our ability to learn new information and the work the brain does to maintain important neural pathways. Sleep spindles—spikes in brain activity that occur during this stage of sleep—are also thought to be essential for committing new skills to memory, such as driving a car or riding a bike. Plus, REM sleep is linked to creativity. Adults spend approximately 20 percent of their night in REM sleep, while infants will remain in the stage as much as 50 percent of the time.

While scientists have known for some time that REM sleep is critical to memory and learning, the connection has been difficult to study since the stage of sleep only lasts in short bursts (about 10 minutes).

To learn more about the link between REM sleep and memory formation, a team of scientists led by researchers at McGill University in Montreal, Quebec, studied mice. They used special light to silence select neurons in the hippocampal region of the brain, which targeted a certain signaling pattern, known as theta oscillations, that scientists know are involved in learning and memory. This special light didn't keep the mice awake but only disturbed the REM sleep stage. The researchers used the light until the mice made it to the next stage of the cycle.

Next, the researchers tried a series of obstacles to test their memory. They found the mice had trouble forming contextual memories—they couldn't remember new places or objects. They also appeared to have trouble processing emotions. The researchers had placed the mice in certain spaces and conducted fear conditioning, which involved shocking their feet, but when placed in the space the following day—after a night of lousy REM sleep—they found the mice didn't associate their surroundings with fear conditioning. These changes in memory didn't occur when researchers disrupted the other stages of sleep in the mice.