What Happens in the Brain During REM Sleep?

Sleep looks passive from the outside. The body is still, the eyes are closed, and the world is temporarily set aside. What is actually happening inside the brain during those hours is something else entirely.

Far from shutting down, the sleeping brain moves through a series of distinct stages, each with its own pattern of activity, its own neurochemistry, and its own relationship to the experiences we carry into and out of sleep. Of all these stages, the one that has attracted the most sustained scientific attention is REM sleep, rapid eye movement sleep, the phase most closely associated with vivid dreaming and, increasingly, with some of the brain's most essential maintenance work.

Understanding what happens during REM sleep does not just satisfy scientific curiosity. It begins to explain why dreams feel the way they do, why they are so emotionally charged, why they are so difficult to hold onto after waking, and why paying attention to them might matter more than most people assume.

The Architecture of Sleep

Sleep is not a single state. It is a cycle, repeating roughly every 90 minutes throughout the night, moving between two broad categories: non-REM sleep and REM sleep.

Non-REM sleep itself has three stages. The first is light sleep, the transitional state between waking and sleeping. The second is characterised by sleep spindles, bursts of neural activity thought to play a role in memory consolidation. The third, slow-wave sleep, is the deepest phase, where the brain generates its slowest and highest-amplitude electrical patterns and where physical restoration is most intensive.

REM sleep follows. In a typical night, the first REM period is brief, perhaps ten minutes. Each successive cycle through the night brings a longer REM phase, so that the final hours before waking are dominated by it. This is why the dreams people remember most vividly tend to be the ones closest to morning (Hobson, 2002).

During REM sleep, the brain's electrical activity shifts dramatically. The slow, synchronised waves of deep sleep give way to fast, irregular patterns that closely resemble waking brain activity. The brain, in a very real neurological sense, is nearly as active as it is when a person is awake and engaged with the world. The body, however, is not. A mechanism called REM atonia temporarily paralyses the skeletal muscles, preventing the sleeper from physically acting out whatever is unfolding in the dream. The eyes move rapidly, giving the stage its name.

Which Parts of the Brain Wake Up

The shift in overall activity level during REM sleep is accompanied by a specific pattern of regional activation and deactivation that directly shapes the character of dreaming.

Several areas become notably active. The amygdala, the brain's primary centre for processing emotion, is highly engaged during REM sleep, more so than during waking in some studies (van der Helm et al., 2011, cited in International Journal of Dream Research, 2025). This activation is directly reflected in the emotional intensity of dreams. Dreams during REM sleep are rarely neutral. They carry fear, longing, excitement, grief, and joy with a vividness that waking thought rarely matches. The amygdala is a large part of why.

The hippocampus, central to memory formation and retrieval, is also active, contributing autobiographical and episodic material to dream content. This is consistent with the finding that people who appear frequently in dreams are overwhelmingly people the dreamer actually knows, and that the settings and situations of dreams, however distorted, tend to have roots in real experience (Domhoff, 2003).

The visual cortex activates in the absence of actual visual input, generating the imagery of dreams from the inside rather than the outside. The motor cortex shows activity, too, though the paralysis of REM atonia prevents it from translating into movement.

What is quieter is equally significant. The dorsolateral prefrontal cortex, the region most associated with rational deliberation, critical thinking, working memory, and the monitoring of logical consistency, is substantially less active during REM sleep (Siclari et al., 2017). This is why dreams feel real as they happen. The part of the brain that would ordinarily flag impossibilities, flying, buildings that change shape, people who are simultaneously two different people, is not running its usual checks. Without that oversight, the dream's internal logic goes unchallenged. Events that would seem obviously strange when awake pass without question in sleep.

The Neurochemistry of Dreaming

The regional pattern of activation during REM sleep is driven by specific shifts in neurotransmitter levels that distinguish it from both waking and non-REM sleep.

Acetylcholine, associated with arousal and neural activation, rises during REM sleep and is thought to be a primary driver of the stage's characteristic brain activity. At the same time, two neurotransmitters that are prominent during waking hours, noradrenaline and serotonin, fall to near-zero levels (Hobson, 2002).

The absence of noradrenaline is particularly significant. Noradrenaline is the neurochemical most closely associated with the fight-or-flight response, the sense of alarm and urgency that accompanies genuine threat. During REM sleep, without it, the brain can replay and process emotionally difficult material, including fear memories, without triggering the same degree of physiological distress that would accompany those experiences while awake. Recent research has described this as a kind of neurochemical safe space: the emotional content is present and active, but the alarm system is turned down (ScienceInsights, 2025). This is one of the mechanisms by which sleep, and dreaming specifically, appears to reduce the emotional charge of difficult experiences overnight.

In people with PTSD, this system is disrupted. Noradrenaline levels during REM sleep do not fall in the way they do in the general population, which may explain why trauma-related nightmares are so persistent and so distressing: the brain is processing the emotional material without the neurochemical buffer that ordinarily makes that processing tolerable (Journal of Neuroscience, 2023).

Memory Replay and the Sleeping Brain

One of the most striking findings in recent sleep neuroscience is that the brain does not simply rest during sleep. It replays.

During slow-wave sleep, the hippocampus replays patterns of neural activity from the day, apparently in the service of transferring recent experiences into longer-term cortical storage. During REM sleep, a different kind of processing takes place. Rather than direct replay, REM sleep appears to support the integration of new material with existing memory networks, finding connections, drawing out general patterns, and stripping away the contextual specificity that ties a memory too rigidly to a single moment in time (Wamsley and Stickgold, 2010).

This is part of why dream content so often combines elements from different times and contexts in unexpected ways. The person from ten years ago appears in a setting from last week. A childhood home has the layout of an office. These combinations are not errors. They may be the visible trace of memory integration at work, the brain building bridges between experiences that share some underlying emotional or structural similarity.

A Brief Bridge to Meaning

The neuroscience of REM sleep tells us a great deal about the mechanism. It explains the emotional intensity of dreams, their narrative looseness, their tendency to blur categories and combine unexpected elements, and their resistance to being remembered. It provides a biological rationale for the fact that sleep disruption reliably impairs emotional regulation, and that people deprived of REM sleep specifically show measurably worse performance on tasks requiring creative thinking and emotional processing.

What the neuroscience does not fully address is the question of content. It can explain why dreams are vivid and emotionally charged. It does not explain why a particular person dreams of a particular image on a particular night, why certain symbols recur across weeks and months, or why a dream can leave behind a feeling that persists through the whole of the following day.

That is where the psychological tradition picks up the thread. Jung (1968) argued that the specific imagery of dreams is not accidental. It reflects the concerns, conflicts, and unresolved tensions of the dreamer's inner life in a symbolic language that is worth learning to read. The neuroscience of REM sleep gives that imagery its stage. Understanding the imagery itself is a different kind of work.

Conclusion

REM sleep is one of the most complex and neurologically active states the human brain ever enters. The amygdala fires, the hippocampus replays and integrates, the visual cortex generates images without any external input, and the prefrontal cortex steps back far enough to let all of it unfold without interference. The neurochemical conditions are unlike any other state, calibrated to allow emotional processing in a way that waking life simply does not permit.

Dreams are not the random noise of a sleeping brain. They are the product of a precisely orchestrated neurological event that the brain has been running, every night, for the whole of human history. Paying attention to what that event produces is not mysticism. It is one of the more direct ways available of understanding what the mind is actually doing when no one is watching.

If you are curious about the meaning behind what your dreaming brain produces each night, Murka is designed to help you explore it. You can begin at murkaverse.com.

References

Domhoff, G.W. (2003) The Scientific Study of Dreams: Neural Networks, Cognitive Development, and Content Analysis. Washington, DC: American Psychological Association.

Hobson, J.A. (2002) Dreaming: An Introduction to the Science of Sleep. Oxford: Oxford University Press.

International Journal of Dream Research (2025) Associative Rehearsal and Reframing Model: A new integrative framework for dreaming. Available at: https://journals.ub.uni-heidelberg.de/index.php/IJoDR (Accessed: 3 April 2026).

Journal of Neuroscience (2023) Emotional memory processing during REM sleep, Journal of Neuroscience, 43(3), pp. 433–446.

Jung, C.G. (1968) Man and His Symbols. New York: Dell.

ScienceInsights (2025) Why do humans dream? What neuroscience reveals. Available at: https://scienceinsights.org/why-do-humans-dream-what-neuroscience-reveals/ (Accessed: 3 April 2026).

Siclari, F. et al. (2017) The neural correlates of dreaming, Nature Neuroscience, 20(6), pp. 872–878.

Wamsley, E.J. and Stickgold, R. (2010) Dreaming and offline memory processing, Current Biology, 20(23), pp. R1010–R1013.