Key Takeaways
We dream during both rapid eye movement (REM) sleep and non-REM sleep.
Reports of dreams collected after rapid eye movement (REM) sleep tend to display greater complexity and connectedness than non-REM dreams.
For the first time, researchers have used graph theory to analyze differences in structural connectedness in REM dreams and non-REM dreams.
We may not know for sure why we dream, but researchers are learning more about what happens while we dream. A recent study, published in PLoS ONE, confirmed that the kind of dreams we have vary depending on what stage of sleep we’re in.
A team from the Federal University of Rio Grande do Norte’s Brain Institute (IC-UFRN) in Brazil analyzed 133 dream reports (from 20 young adult volunteers) word by word using graph theory, a branch of analysis that focuses on networks and other kinds of relationships among words or other items in a set. The volunteers were woken up during rapid eye movement (REM) sleep, when dream reports are most plentiful, and also during non-REM sleep, which produces fewer dream reports.
Previous research has found that REM dreams are longer, more expressive, and more “movie-like” than non-REM dreams. But this study is the first of its kind to prove this by using graph theory to measure the structural differences in dreams.
A Novel Approach
The researchers developed a tool that can analyze a large amount of data, at high speed and without subjective biases, such as linguistic bias.
Because the tool works in any language, it successfully identified changes in dream quality and complexity between sleep stages by analyzing the way the dreamer communicates rather than differences that depend on semantics. And the results are important because they show that computational methods can be applied to studies of dreaming.
In other words, graph theory doesn't analyze how strange a dream is or what the content might mean. What’s important is how subjects organize and recount their memories of the dream. The researchers believe the word graph tool is a valuable addition to knowledge of dreams and can be used for a broad analysis of dream reports worldwide, such as those in the online repository Dream Bank, which currently holds more than 20,000 reports.
REM sleep likely includes the complex activation and interaction of different areas of the brain, leading to a more elaborate dream experience.
Graph theory is a very novel approach to analyzing dreams, says Rebecca Spencer, PhD, professor of psychological and brain sciences at University of Massachusetts Amherst. "I think the advance for the sleep field will now be to apply this tool to various situations, perhaps to see what N3 dreams look like or to see what dreams of different populations look like–imagine comparing this difference between N2 and REM dreams for young adults as reported here to those of older adults who some say have more vivid dreams.
Spencer also thinks the study is interesting from the perspective of understanding consciousness. "We have come to think of sleep and wake as two separate states of consciousness, and it's interesting to think of this as support for each sleep stage being a unique state of consciousness," she says.
A Quick Recap on Sleep Stages
We divide sleep into REM and non-REM sleep–and alternate between both throughout the night – but there’s a lot more to it than that. “Non-REM sleep includes stage 1 (N1), stage 2 (N2), and stage 3 (N3) sleep, correlated with increasing depth,” says Brandon Peters-Mathews, MD, a board-certified physician in both neurology and sleep medicine who currently practices at Virginia Mason Medical Center in Seattle.
The non-REM stages are described as follows:
N1 sleep is the lightest state of sleep and is often misinterpreted as wakefulness.
N2 sleep makes up about half of a typical night and is characterized by the presence of sleep spindles and K complexes, electrical patterns generated deep within the brain in an area called the thalamus.
N3 sleep includes high-amplitude, slow waves and is the most difficult state from which to wake a person.
REM often occurs at 90- to 110-minute intervals through the night, with more of it concentrated in the last third of the night as the REM periods become more prolonged.4 This means it's common to wake from REM in the morning.
REM vs. Non-REM Dreams
Dreams may occur in both REM and non-REM sleep. "Early in the night, you hit N3, when we know memories are replayed–it's like taking the video of your day and putting it on repeat," says Spencer. "So if you are woken up during this stage, you are likely to have 'dreams' that sound like memories from earlier. While we know less about N2 dreams, likely a similar thing is happening.
Dreams in non-REM sleep are often simple ideas, whereas REM-related dreams may be complex, immersive experiences—Dr. Peters-Mathews likens a non-REM sleep dream to a picture, while an REM dream is a movie in which the dreamer is an actor. “REM sleep likely includes the complex activation and interaction of different areas of the brain, leading to a more elaborate dream experience,” he says.
"We have long thought that when you go into REM sleep, those recent memories trigger more remote memories and perhaps by getting randomly initiated, you get the crazy combination of ideas that we think of as REM dreams," Spencer says.
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