What do the stages of sleep reveal about consciousness?
By Rachel Barr | Published: 2025-08-20 14:30:00 | Source: Neuropsych – Big Think
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Every night, without protest or panic, we cease to exist as subjects of experience. One moment, we are here, sensing and responding. Next, we were gone. This phenomenon is so familiar that few of us stop to question it, but if you want to understand consciousness, there is no better place to look than in sleep and dreaming.
In 1974, philosopher Thomas Nagel asked:What’s it like to be a bat?“This question became a landmark in the philosophy of mind precisely because it defined consciousness by its most important property: what it is like. If experience is present, so is consciousness.
The problem is that this formula is difficult to understand, and even more difficult to implement. It is one thing to say that experience lies at the heart of consciousness, but quite another to actually study it. Bats, after all, don’t answer questionnaires.
Consciousness resists external analysis because its nature is internal. We may not know what it’s like to be a bat, but every night, we face the essential question: What’s it like to be anything at all?
When we are awake, conscious experience is shaped by sensation and action. Streams of sensory input flow inward, motor outputs flow outward, and this constant exchange with the environment anchors our subjective experience. But when we sleep, that architecture disintegrates and rearranges itself.
The stages of sleep are divided into two broad stages: non-REM and REM. Neuroscientists can track these conditions and their distinct neural signatures using electroencephalograms (EEG), which record the collective electrical activity of cortical neurons. In the deeper stages of non-REM sleep, these neurons fall into a slow, rhythmic oscillation of activity interspersed with periods of silence. To be in one of these slow waves, subjectively, is to be nowhere at all. Studies using high-density EEG They have shown that when slow wave activity dominates posterior cortical areas, conscious experience is usually absent.
We fall into non-REM sleep and we know what it means, and then, nothing. Complete annihilation of Ho-ness.
Only in the dream-laden rhythms of REM sleep does consciousness reappear, and we return with a sense of what it was like to disappear.
On the surface, REM sleep looks like rest. Motor command centers in the brainstem inhibit muscles, leaving them temporarily paralyzed, and their responsiveness to the outside world diminishes. But inside the brain, the lights are on. Rapid, asynchronous shooting It reappears in the posterior cortex, with levels of activity in sensory and emotional circuits matching — and in some areas even exceeding — those we see during the day. Our eyes are closed, yet the visual cortex shows activation patterns similar to what it does when processing a real scene.
What emerges are dreams – internally generated loops of consciousness, which are constructed in the perceptual void. The other missing element is metacognition – awareness of awareness – and a sense of power. In REM, we are undoubtedly conscious. Completely immersed, but without the ability to watch ourselves from outside the moment. Most dreamers do not know they are dreaming; They simply exist in the experience. To dream is to be without knowing that you are.
Lucid dreaming provides a counterexample. In rare cases, sleepers gain insight into the unreality of their dreams from within the dream itself. In sleep labs, researchers train dreamers to signal lucidity through intentional eye movements, which can be time-stamped using electrooculography (EOG). Functional MRI and EEG data He and colleagues showed that the shift to lucidity coincides with a partial reactivation of the dorsolateral prefrontal cortex, anterior cingulate cortex, and precuneus, regions that support metacognition and are typically offline during REM sleep. When a dream becomes lucid, it is not the experience that changes, but the context around it. The lights were on all the time, and now, you know it.
Ordinary dreams are perhaps the clearest expression of what it means. Instead of defining subjectivity from the outside, REM Sleep allows us to map its loss and return from withinIt reveals its structure by deconstructing it. The dream becomes a living rehearsal of Nagel’s challenge: Consciousness is neither something we do nor a behavior we emit. It is a state of self-sustained inner activity, determined by the quality of felt experience.
What remains when our conscious experience is stripped of memory, agency, and self-reflection? What’s it like to be created without them? We may never know exactly, but we can step outside of our dominant perspective to understand what it means to be someone – or something – in a fundamental sense. In sleep, we are close to the inaccessible Nagel Patience As we probably will.
This article is part of the special issue of Consciousness. Read the entire collection here.
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