Hallucinations can only occur when we're awake… Right? By definition, probably; in experience, probably not. Since antiquity, philosophers and scientists have been overwhelmed by the perceptual phenomena that appear on the fringes of consciousness, and among the few points of agreement is the existence of an unconscious state of mind capable of generating "false realities," yet surprisingly convincing ones: dreams.

The French researcher Alfred Maury observed in the 19th century that there was a continuous spectrum between dreams and hallucinations, both in their form and their causes. Over time, various studies on disorders such as schizophrenia showed sleep disturbances in patients who also experienced hallucinations. This led many researchers to believe that dreams and hallucinations might share a common root, perhaps related to the intrusion of REM sleep mechanisms (the kind associated with vivid dreams) into waking life. In other words, speculation began that certain hallucinations were nothing more than fragments of dreams that intrude into waking consciousness. Even so, most medical manuals continued—and still continue—to sharply distinguish between hallucinations typical of wakefulness and those that occur during sleep.

Within this mysterious territory lie hypnagogic hallucinations. The term hypnagogic comes from the Greek words hypnos, meaning sleep, and agogos, meaning leader or guide; literally, "that which leads to sleep." Therefore, this type of hallucination consists of spontaneous and vivid perceptions that arise as one is falling asleep. They usually appear involuntarily, beyond the person's control, and can be accompanied by intense emotions, ranging from curiosity to fear. The content of these experiences is usually visual, such as geometric figures, animals, faces, or human figures. However, auditory elements are also common, including voices, footsteps, or even pieces of music.

Regarding their prevalence in the population, some studies indicate that up to 70% of people have experienced them at least once in their lives, although only a minority experience them frequently. Furthermore, most people experience them during periods of stress, fatigue, or sleep disturbances, although for a few they can recur often enough to cause distress.

Currently, experts classify hypnagogic hallucinations as transitional phenomena, that is, a kind of waking dream caused by the overlap of the neural systems of sleep and wakefulness. This is because, at the onset of normal sleep, the brain regions responsible for sensory processing and voluntary movement begin to shut down, while the circuits that generate dreams gradually activate. When the coordination between these systems fails, dream images can intrude into consciousness before the rest of the brain has "fallen asleep." Thus, the hypnagogic experience is not a sign of any disorder, but rather a disruption in the brain's highly complex choreography during the transition from wakefulness to sleep.

From a psychological and philosophical perspective, these experiences raise fascinating questions about the nature of consciousness. They reveal that perception doesn't simply switch off like a light, but rather wavers as one neural network gives way to another. In a sense, the brain in transition is simultaneously awake and dreaming. Some neuroimaging studies show that the visual and auditory cortices remain active during this phase, although their activity becomes decoupled from external stimuli and dominated by internal signals.

However, before neuroscience could explain hypnagogic hallucinations, they were often interpreted as supernatural encounters. What more convincing explanation could there be than the idea that a shadow from the underworld came to visit you in the darkness to whisper some of its secrets?

Numerous artists and inventors have harnessed these secrets as a source of inspiration. It is said that Thomas Edison and Salvador Dalí used hypnagogic states to generate ideas. For example, there is a myth that they would hold metal objects in their hands so that, as they fell asleep and their muscles relaxed, the sound of them falling would awaken them just as the images began to emerge. They would then jot down these fleeting visions before they vanished. Similarly, it is said that one day, when the chemist August Kekulé dozed in front of the fireplace, he visualized a snake biting its tail and thus understood the ring structure of benzene, something that until then had been a mystery to the scientific community. Currently, some researchers are studying this same liminal phase as a window into the creative process, a moment when logical filters weaken and the imagination is unleashed.

In conclusion, the difference between sleep and wakefulness cannot be explained as a switch that turns on and off, because it is such a complex spectrum that, in some circumstances, it can show its flaws, and it is then that one discovers the fragility of what we have always called "reality".

The original article was published in Spanish at Ethic.es