Dreams Beyond Rem Sleep: What Are They Like?

Dreams can occur during both REM and non-REM sleep. However, the dreams that occur during REM sleep tend to be more vivid and memorable. During non-REM sleep, dreams are more conceptual and less vivid.

Dreaming in non-REM sleep is less frequent and less vivid

During non-REM sleep, the brain is less active than during REM sleep. In the deeper stages of non-REM sleep, breathing slows down and blood pressure drops.

Dreams during non-REM sleep are more difficult to recall. In a study, participants were awakened during non-REM sleep and asked if they were dreaming. These participants reported fewer dreams, which were more conceptual and less vivid and emotional than dreams during REM sleep.

Another study found that dreams during non-REM sleep tended to be more abstract than dreams during REM sleep, which were more elaborate and followed a narrative.

However, dreams can occur during any stage of sleep.

Non-REM dreams are more conceptual and less emotional

During REM sleep, your eyes move rapidly, your brain is active, and dreams typically occur. In contrast, during Non-REM sleep, your brain is less active, and in the deeper stages, your breathing slows down and your blood pressure drops. After falling asleep, Non-REM sleep comes first, followed by a shorter period of REM sleep, and the cycle repeats.

Although dreams occur in both REM and Non-REM sleep, there are differences in the nature of dreams during these two stages. Dreams during Non-REM sleep are more conceptual and less vivid and emotional than dreams during REM sleep. Non-REM dreams are also less elaborate and less story-like, often consisting of isolated visual imagery or conceptual, non-visual experiences.

Recall of Non-REM dreams

Non-REM dreams are harder to recall than REM dreams. When awakened during Non-REM sleep, people are less likely to report having had a dream and, when they do, the dream is often recalled as a "white dream", where the feeling of dreaming is remembered but not the content.

Characteristics of Non-REM dreams

Non-REM dreams are associated with fewer, smaller, and shallower slow waves and faster spindles in the brain, especially in central and posterior cortical areas. These dreams are more likely to occur when slow waves are sparse, small, and shallow, particularly in the posterior and central brain regions.

High-amplitude slow waves in Non-REM dreams

A small subset of Non-REM dreams are associated with very steep and large slow waves in the frontal regions of the brain. These large slow waves are accompanied by high-frequency power increases or "microarousals", which may facilitate dream recall.

Type I and Type II Non-REM slow waves

Non-REM slow waves can be classified into two types: Type I and Type II. Type I slow waves are large and steep with frontocentral involvement, while Type II slow waves are smaller, shallower, and more diffusely distributed. Dreaming is more likely to occur when there is a higher ratio of Type I to Type II slow waves.

Spindles in Non-REM dreams

Non-REM dreams are associated with a higher number of fast spindles and fewer slow spindles in the central and posterior regions of the brain.

Non-REM dreams are more likely when slow waves are sparse, small and shallow

Dreaming can occur during both REM and non-REM sleep. However, dreams during non-REM sleep are less frequent and are often not recalled. When they do occur, non-REM dreams are more conceptual and less vivid, intense, bizarre, and emotional than dreams during REM sleep.

A 2018 study found that non-REM dreams are more likely when slow waves are sparse, small, and shallow. In this study, researchers used high-density EEG recordings and a serial awakening paradigm to investigate how specific features of slow waves and spindles, the hallmarks of non-REM sleep, relate to dream experiences. They found that reports of dreaming, compared to reports of no experience, were preceded by fewer, smaller, and shallower slow waves, especially in central and posterior cortical areas.

The study also identified a small subset of very steep and large slow waves in frontal regions, which occurred on a background of reduced slow-wave activity and were associated with high-frequency power increases (local "microarousals") that heralded the successful recall of dream content. These findings suggest that the capacity of the brain to generate experiences during sleep is reduced in the presence of neuronal off-states in posterior and central brain regions, and that dream recall may be facilitated by the intermittent activation of arousal systems during non-REM sleep.

Non-REM dreams are more likely when high-frequency power increases occur

Dreams during non-REM sleep are more likely to occur when there is an increase in high-frequency power. During non-REM sleep, the brain is less active compared to REM sleep, and the body enters a state of deeper sleep. While dreams typically occur during REM sleep, they can also happen during non-REM sleep, but they tend to be less vivid and emotional, and more abstract and conceptual.

High-frequency power increases in the brain during non-REM sleep can lead to an increased likelihood of conscious dream experiences and lucidity. This suggests that brain activity during non-REM sleep dreams may share similarities with brain activity during REM sleep dreams, where high-frequency power is also present.

Research has shown that during both REM and non-REM sleep, dreaming is associated with local decreases in slow-wave activity in posterior brain regions. This decrease in slow-wave activity could be a neural correlate of dreaming, indicating that certain brain regions are less active during dream experiences.

Additionally, studies have found that individuals awakened from REM sleep reported more vivid and emotional dreams, while those awakened during non-REM sleep reported less frequent and less intense dreams. This suggests that the brain activity associated with dreaming may be different during non-REM sleep, resulting in a lower likelihood of dream recall upon awakening.

Overall, the increase in high-frequency power during non-REM sleep can create the conditions for more conscious and lucid dreams, but the dreams themselves may be less elaborate and emotional compared to those experienced during REM sleep.

Non-REM dreams are more likely when there are more fast spindles

Sleep occurs in two main stages: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further divided into three substages. During the first stage, you are in a light sleep and can be woken up easily. In the second stage, your body temperature drops, and your heart rate and breathing slow down even further. The third stage is the deep sleep stage, where it is harder to wake you up, and your body repairs and regenerates tissues, builds bone and muscle, and strengthens your immune system. After NREM sleep, you enter REM sleep, during which your eyes move rapidly, your brain activity increases, and you experience intense dreams.

Dreams can occur during any sleep stage, but the vivid dreams that you remember tend to happen during REM sleep. However, studies have shown that dreams can also occur during NREM sleep, particularly in its second stage. These dreams tend to be more conceptual and abstract, lacking the vividness and emotional intensity of REM dreams.

Sleep spindles are a specific pattern of brain waves that people experience during certain sleep stages. They are identified by electroencephalography (EEG), which measures electrical activity in the brain. Sleep spindles generally occur during NREM sleep, specifically during the second stage, and they do not occur during REM sleep. Sleep spindles can be categorised into two types: fast spindles and slow spindles, which are differentiated by their frequencies. Fast spindles have a frequency higher than 13 Hz and are found in the centroparietal region of the brain, while slow spindles have a frequency lower than 13 Hz and are more prevalent in the frontal region of the brain.

While the purpose of sleep spindles is not yet fully understood, they are thought to play a role in brain plasticity and the process of learning and integrating new memories. They also appear to diminish our response to external stimuli while sleeping. Research has shown that sleep spindles occur as a result of activity in the thalamus, the thalamic reticular nucleus, and the neocortex.

One study found that the rate of sleep spindles increased significantly before the onset of REM sleep. This suggests that sleep spindles may play a role in regulating sleep-wake states and facilitating the transition from NREM to REM sleep. Additionally, the study found that optogenetic activation of specific neurons or their projections in the thalamus increased the occurrence of sleep spindles and the probability of transitioning to REM sleep.

In summary, while dreams during NREM sleep are less common and less vivid than those during REM sleep, they can still occur. The occurrence of dreams during NREM sleep, especially in its second stage, has been documented in various studies. The presence of sleep spindles, particularly fast spindles, during NREM sleep may be a contributing factor that increases the likelihood of dreaming during this stage.

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