Scientists have a new theory about REM sleep
This post originally appeared on Salon by Matthew Rozsa.
During the quiet third of our lives that we spend sleeping, the human body is doing something that may not seem restful at all: REM sleep, short for “rapid eye movement,” is a phase of sleep that consumes 90 to 120 minutes of sleep. the day for an adult human and up to nine full hours for a newborn. In this phase of sleep, your eyes twitch randomly and repeatedly, and sleepers have their most vivid dreams; people awakened from REM sleep often feel like they actually happened. Scientists note that parts of the neocortex, associated with higher thought forms, begin to activate seemingly at random.
Although REM is only a minority constituent of time spent sleeping, it is perhaps the most enigmatic stage. What is the purpose and function of REM sleep, and why we do it, remains a mystery.
Now, a new study published in the scientific journal Neuron suggests REM sleep may have evolved to help protect us from predators. In other words, it’s a holdover from an earlier stage of human evolution, in which hominids had to be on the lookout for danger everywhere, even – perhaps especially – at night.
Dr. Wang Liping of the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences led a research team that placed animal subjects in a sealed chamber and monitored their brain activity while they slept. In order to simulate the feeling of believing that a predator is nearby, they exposed the animals to the smell of trimethylthiazoline, which is similar to the smell of a predator. By doing this to different animals at different stages of their sleep cycles, they were able to compare how quickly the animals woke up from sleep depending on which stage they were in. It turned out that the animals woke up more quickly if they were in an REM cycle than if they were in an NREM cycle (not REM).
Scientists also found something interesting in the brains of animals that were exposed to a “predator” during their sleep REM cycle. Neurons in a region of the brain called the medial subthalamic nucleus, which produce a stress-associated hormone called corticotropin, gave their animal hosts a lower threshold for awakening than animals sleeping in NREM mode. These animals were also more likely to have highly defensive responses after being excited.
“Together, our findings suggest that adaptive responses to REM sleep may protect against threats and uncover an essential component of neural circuitry at their base,” the authors conclude. Their findings have implications for the treatment of mood disorders and other conditions that may be linked to a neurological link between sleep and fear.
This isn’t the first study to link REM sleep to defense against predators. An article from 2013 in the magazine Dream by Ioannis Tsoukalas of Stockholm University in Sweden hypothesized that many physical states associated with REM sleep are similar to tonic immobility, or the state in which animals pretend to be dead and appear therefore unattractive to predators which they cannot fight or outrun. Tsoukalas notes that people in REM sleep cannot move, which is similar to how some animals freeze when frightened, and people in REM sleep also share traits of tonic immobility like breathing and frequency impaired heart rate, impaired thermoregulation, suppression of reflexes and even extra “theta”. waves in EEG patterns (these are derived from the hippocampus and are related to spatial awareness and memory). According to this hypothesis, even the intense dreams we experience during REM sleep could simply be our brains sorting out potential threats.
REM sleep has many other reasons for being. Scientists have shown that REM sleep is linked to consolidation spatial and contextual memories, and it’s generally accepted that babies have more REM sleep than adults because their brains are at a very formative stage of development. REM sleep is also linked to increased creativity, with an article from 2018 in the magazine Trends in cognitive science put forward a new theory: that NREM sleep is a period in which the brain begins a problem-solving process by separating important information from mere noise, then REM sleep completes it by abstractly searching for this information to find possible connections .