Sleep laboratory 06 · Believing the impossible
The Dreaming Brain and Reduced Reality Testing
A room changes into a railway station, a dead person is alive, and gravity stops applying. Most dreamers accept such events without surprise because the systems that generate experience remain active while reflective checking is reduced.
Three core signals
At a glance
SIGNAL 01
Immersion
The dream is experienced from inside rather than inspected as a hypothesis.
SIGNAL 02
Monitoring
Reflective and metacognitive control is often weaker than in waking.
SIGNAL 03
Lucidity
Reality testing can partially return while the dream continues.
Research note 01
What Reality Testing Involves
In waking life, beliefs are corrected by stable sensory evidence, memory, social feedback and deliberate checking. Source monitoring helps distinguish perception from imagination and recall.
During dreams, the sleeper has limited access to the external environment and often reduced access to a stable autobiographical perspective. Contradictions therefore receive less challenge.[1]
Research note 02
Prefrontal Changes Without a Simple Off Switch
Some lateral prefrontal and parietal systems involved in working memory and metacognition are less active during ordinary REM sleep than waking. Meanwhile, emotional and perceptual systems remain active.[1], [2]
It is inaccurate to say that the prefrontal cortex is completely off. Activity varies by region, sleep stage and moment, and dreams can include planning, self-reflection and deliberate choice.
Reality Testing Is Graded
Dream awareness is not divided neatly into “rational waking” and “irrational sleep.” Ordinary dreams vary in self-reflection, memory and control. Some dreamers question an odd detail without becoming lucid; others know they are dreaming but exercise little control over the scene.
Lucid dreaming provides the clearest comparison because insight returns while sleep continues. A review of EEG, imaging, lesion, pharmacological and stimulation studies found that the evidence remains limited: EEG studies are often underpowered and mixed, while preliminary imaging points to prefrontal and parietal involvement.[3] The careful conclusion is partial restoration of metacognitive function, not a fully awakened executive system.
This graded view explains why a dream can contain pockets of good reasoning. A person may solve a problem, make a plan or remember a waking goal while still accepting an impossible setting. The relevant capacities can separate rather than switching on together.
It also protects against a common error. Dream credulity is not evidence of poor intelligence or weak critical thinking. The same person who rejects an impossible claim while awake may accept it in a dream because access to sensory correction, working memory and autobiographical context has changed.
Research note 03
Why Bizarreness Is Tolerated
A dream can preserve local coherence while violating global reality. Each scene supplies its own assumptions, and memory for what came before may be unstable. Emotional plausibility can replace physical plausibility.
Reduced sensory prediction error also matters: an impossible dream object is not corrected by a stable view from the open eyes.
Not all dreams are bizarre. Many reproduce ordinary rooms, conversations and concerns. Bizarreness is also partly a property of reports: a long account has more opportunities to contain an odd transition than a short fragment.
Research note 04
Lucid Dreaming as a Comparison Case
In a lucid dream, a person recognises that they are dreaming while sleep continues. Lucidity is associated with partial restoration of metacognition and possible changes in frontoparietal activity or connectivity.[3]
Lucid dreams show that reality testing is graded, not simply present or absent. They also allow trained participants to signal from sleep using pre-agreed eye movements.
A Conversation from Inside REM Sleep
Lucid dreamers can sometimes do more than signal that lucidity has begun. In a four-laboratory study, experimenters communicated with 36 participants during polysomnographically verified REM sleep. Questions were delivered through speech, flashing lights or touch, and participants replied with pre-agreed eye movements or facial-muscle contractions.[4]
Correct answers were documented on 29 occasions across six participants.[4] In one case, a spoken arithmetic problem was heard within a video-game dream. In another, flashes used to transmit a problem appeared as flickering objects in the dream world. These are rare successes, not routine two-way interviews, but they demonstrate that external information can sometimes be perceived, held briefly in working memory and answered without ending the dream.
The experiment also reveals how reality testing can be selective. A participant may know that an external researcher is asking a question while still experiencing the surrounding dream as a convincing place. Lucidity adds a higher-level belief – “this is a dream” – without necessarily dissolving the imagery.
The failures matter as much as the successes. Many questions received no response, some answers were ambiguous and later reports did not always match the recorded exchange. Real-time communication therefore narrows uncertainty; it does not eliminate the problems of sampling, recall and interpretation.
Source Monitoring Inside a Self-Generated World
During waking, source monitoring asks whether information came from perception, memory, imagination or another person. A dream complicates the task because much of the scene is internally generated but experienced as external. A voice can feel as if it belongs to a character even when it incorporates a real sound from the room.
Lucidity shows that source monitoring can return in layers. The dreamer may correctly identify the state as a dream, correctly attribute a flashing light to the laboratory and still misunderstand the content of the signal. This is more informative than saying the “logic centre” is off. Different forms of monitoring can be preserved, weakened or restored separately.
The distinction also separates ordinary dreaming from psychosis. Both can involve experiences not constrained by current external stimuli, but a sleeping dream is a normal state-bound event that usually resolves on awakening. Clinical hallucinations and delusions occur in waking contexts and require their own assessment.
What the Evidence Supports
| Claim | Current standing | Reason |
|---|---|---|
| Metacognition is often reduced in ordinary dreams | Well supported | Reports and neuroimaging converge on weaker reflective monitoring.[1], [2] |
| The entire prefrontal cortex switches off | False | Changes are regional and relative, not total. |
| Dream bizarreness results from one inactive region | Too simple | Sensory disconnection, memory instability, emotion and network changes all contribute. |
| Lucidity restores some reality testing | Supported | Lucid dreamers recognise the state and can sometimes communicate deliberately.[3], [4] |
Interpretive Boundaries
Dream neuroscience is most informative when it states clearly what a method measures and where inference begins. The following boundaries prevent an interesting finding from becoming an exaggerated claim.
- Dream credulity is not evidence of low intelligence.
- Not all dreams are bizarre.
- Neural activation differences do not map one-to-one onto complex faculties.
- Lucidity is a special state and should not be used to generalise about all dreams.
Source desk
Key Research and Further Reading
- Nir and Tononi, Dreaming and the Brain.
- Baird et al., The Cognitive Neuroscience of Lucid Dreaming.
- Konkoly et al., Real-Time Dialogue Between Experimenters and Dreamers During REM Sleep.
- Siclari et al., The Neural Correlates of Dreaming.
- Scarpelli et al., Investigation on Neurobiological Mechanisms of Dreaming in the New Decade.
Research changes as methods improve. Links are provided so that readers can distinguish direct findings, reviews and theoretical interpretation.
References
- Nir Y, Tononi G. Dreaming and the brain: from phenomenology to neurophysiology. Trends Cogn Sci. 2010;14(2):88–100. doi:10.1016/j.tics.2009.12.001. PMID: 20079677.
- Scarpelli S, Alfonsi V, Gorgoni M, Giannini AM, De Gennaro L. Investigation on neurobiological mechanisms of dreaming in the new decade. Brain Sci. 2021;11(2):220. doi:10.3390/brainsci11020220. PMID: 33670180.
- Baird B, Mota-Rolim SA, Dresler M. The cognitive neuroscience of lucid dreaming. Neurosci Biobehav Rev. 2019;100:305–323. doi:10.1016/j.neubiorev.2019.03.008. PMID: 30880167.
- Konkoly KR, Appel K, Chabani E, et al. Real-time dialogue between experimenters and dreamers during REM sleep. Curr Biol. 2021;31(7):1417–1427.e6. doi:10.1016/j.cub.2021.01.026. PMID: 33607035.
- Siclari F, Baird B, Perogamvros L, et al. The neural correlates of dreaming. Nat Neurosci. 2017;20(6):872–878. doi:10.1038/nn.4545. PMID: 28394322.
The measured night
In Summary
Dreams become believable because internal simulation continues while external correction, stable memory and metacognitive checking are altered. The impossibility is often visible to the waking reader, but not to the mind living inside the scene.