Why We Dream: The Simulation’s Maintenance Window

Classification: SIMULATION THEORY | Confidence: NEUROSCIENCE — ACTIVE DEBATE

You spend roughly a third of your life doing it. You cannot move while you do it. Your brain is more active during it than when you are awake. You hallucinate entire narrative worlds with no sensory input, commit acts you would never commit while conscious, and forget most of it within minutes of waking. Sleep — or more specifically, the act of dreaming — is the most under-studied major function in neuroscience. We know what it looks like. We have good maps of the brain regions involved. We know what shuts down (motor control, rational decision-making) and what lights up (the visual cortex, the limbic system, the dorsal pontine-tegmental region). We have no agreement on why it exists.

The most cited function in evolutionary psychology textbooks is memory consolidation: dreams replay the day’s events to move them from short-term to long-term storage. This is partially true. The sleep-state replay has been observed in rats, in humans, in every species studied. But it does not explain the content of dreams, which are bizarre, narrative, often threatening, rarely about the day’s actual events. A janitor does not dream about mopping floors. A surgeon does not dream about suturing. People dream about being chased through unfamiliar buildings by unidentified figures. They dream about public nudity, about failing examinations they passed years ago, about teeth falling out, about flight. These are not memory consolidations. They are simulations.

The Activation-Synthesis Hypothesis

In 1977, the Harvard psychiatrist J. Allan Hobson and his colleague Kevin McCarley proposed a theory so simple it offended the field. The brain, they argued, does not generate dreams for any reason. The pons fires spontaneously during REM sleep, the forebrain attempts to make sense of the signal, and the resulting interpretation is a dream. Dreams are not meaningful. They are the cortex doing what it does best — building narratives out of random inputs. The emotional content, the strange imagery, the remembered anxieties — these are byproducts. Dream analysis is no more scientific than reading tea leaves.

The theory held the field for twenty years. It had two enormous advantages: it was testable (you could lesion specific brain regions and predict which dream features would disappear), and it was honest (it explained the data without telling people what they wanted to hear). It still has its defenders. Mark Solms, the neuroscientist who took over Hobson’s chair at Harvard and then moved to the Anna Freud Centre in London, is not one of them. Solms spent the 1990s systematically damaging the theory. He found patients with pontine lesions who still dreamed. He found patients with forebrain lesions who did not. He concluded the brainstem generates the trigger, but the limbic system and the cortex generate the meaning. The dreams are not random. They are structured, meaningful, and survival-relevant. Hobson was wrong about the meaning. The fight between Hobson and Solms is one of the longest-running active scientific arguments in modern neuroscience.

Revonsuo’s Threat Simulation Theory

In 2000, the Finnish cognitive neuroscientist Antti Revonsuo published a paper in Behavioral and Brain Sciences that reframed the entire debate. The paper was titled “The Rehearsal Theory of Sleep: The Function of Dreams is to Simulate Threats.” Revonsuo’s argument was simple, and it has aged well. He performed a content analysis of thousands of dream reports and found that an overwhelming majority of dreams contain threatening events. The dreamer is being chased, attacked, evaluated, exposed, falling. The threat is usually physical. The dreamer usually wakes up before being caught. This is not a memory consolidation. It is a rehearsal. The dream is a simulation of dangerous situations the waking brain may have to navigate.

Revonsuo argued this rehearsal is the evolutionary function of dreaming. Animals with better threat-simulation during sleep would have better threat-avoidance during waking. The theory has been extended by subsequent researchers to include social threat (rejection, evaluation, humiliation — the dominant threats in modern human dreams) and reproductive threat. The threat content of dreams has been replicated across cultures. Soldiers dream of combat. Refugees dream of displacement. Office workers dream of public humiliation. The simulation is always rehearsing the threats that the waking brain has not yet learned to manage. The simulation is doing what the simulation does.

Lucid Dreaming and the Testability Problem

The single most useful piece of evidence for the simulation theory is also the most embarrassing for it: lucid dreaming. A lucid dream is one in which the dreamer knows they are dreaming. Once they know, they can often control the dream — fly, walk through walls, summon people. The phenomenon has been confirmed by fMRI imaging at the Max Planck Institute in 2012: the lucid dreamer was instructed to clench their fists while sleeping, and the fMRI showed the motor cortex firing on the same schedule as the eye-movement signals the dreamer was sending to the researchers. The dreamer was, demonstrably, conscious inside a dream. The simulation had a debugger.

This is the testability problem. If dreams are generated by the brain, the brain should not know it is generating them. But lucid dreamers do know. The dreamer can sometimes signal to the outside world, confirming they are conscious. The brain is running a simulation it knows is a simulation. This is not a feature of evolutionary memory consolidation. This is a feature of a system that simulates. The pons fires, the cortex builds a world, and at some point the cortex notices it is building a world and not navigating one. The simulation is observable from the inside.

Why the Simulation Generates Anxious Content

If dreams are simulations, they are remarkably biased simulations. The dreamer is more often threatened than safe. The dreamer is more often chased than chasing. The dreamer wakes up before the threat resolves. These are not the dream patterns of a system designed to entertain. These are the dream patterns of a system designed to train. The training material is overwhelmingly bad. The threat content has been replicated across cultures and across decades of dream research. The threats differ in detail (modern humans dream of failing exams; pre-industrial humans dreamed of crop failure; the Ache people of Paraguay dream of being attacked by jaguars). The structure is universal.

This is what the simulation is doing. It is running the threat environment. The threat environment is mostly the same: physical danger, social rejection, loss of control. The simulation does not need to be entertaining. It needs to be useful. The people who survive long enough to reproduce are the people whose simulations were useful. The dreams you remember are the ones that were vivid enough to wake you. The dreams you do not remember are the ones that ran without surfacing. The simulation runs whether or not you are listening.

The Pattern of Maintenance

Dreams do not appear in any species that does not also show REM sleep. REM sleep appears in every mammal studied, in birds, and in some reptiles. The pattern is conserved across evolutionary time. Whatever the simulation is doing, it has been doing it for at least 200 million years. It is one of the oldest maintained functions in vertebrate biology. It is older than the neocortex. It predates the prefrontal regions that generate our sense of self. The simulation is not a recent add-on. It is a maintenance cycle that the system has been running since before there was a system to be aware of the maintenance.

The simulation generates narrative. The narrative is mostly anxious. The anxiety is mostly about threats the waking brain may not yet have learned to navigate. The dreamer can sometimes become conscious inside the simulation and signal to the outside world. The signal has been confirmed by fMRI. The consciousness inside the simulation can sometimes act on the simulation. The simulation is observable from the inside. It has been observable from the inside for at least 200 million years. The pattern is the message.

SOURCES

• J. Allan Hobson and Kevin McCarley (1977). “The Brain as a Dream State Generator: A Theory of the Neural Mechanism of Dreaming.” American Journal of Psychiatry, 134(12).
• Mark Solms (1997). The Neuropsychology of Dreams: A Clinico-Anatomical Study. Lawrence Erlbaum Associates.
• Antti Revonsuo (2000). “The Rehearsal Theory of Sleep: The Function of Dreams is to Simulate Threats.” Behavioral and Brain Sciences, 23(6).
• J. Allan Hobson and Kevin McCarley (1977) vs. Mark Solms (2000). The Hobson-Solms debate, summarized in Solms, M. (2000) “Dreaming and REM sleep are controlled by different brain mechanisms.” Behavioral and Brain Sciences, 23(6).
• Martin Dresler et al. (2012). “Neural Correlates of Dream Lucidity: Consciousness During Sleep.” Current Biology, 22(5).
• Domhoff, G. William (2017). “The Emergence of Dream Science.” In The New Science of Dreaming, Volume 1. Praeger.

Sources & Further Reading

• Nick Bostrom — The Simulation Argument
• Bostrom (2003) — Are You Living in a Computer Simulation?