The science of nightmares

It turns out “sleeping with one eye open” is more than a figure of speech. Neuroscientists have documented a first-night effect: when you crash in an unfamiliar bed, one hemisphere of your brain actually stays half-awake, as if on sentry duty^[1]. This primitive vigilance may explain why travel insomnia is real, and perhaps why dreams feel stranger in strange places. But nightmares themselves are far more than a quirk of novel surroundings. For perhaps 3-7% of people, recurrent terrifying dreams are a real and ongoing affliction^[2][3]. Even casual users of the word “nightmare” admit it’s typically hyperbole, yet clinically, nightmares are defined as vivid REM dreams that awaken the sleeper with fear or terror^[4]. In sleep labs, people describe leaping from bed at dawn, heart hammering, gasping for breath as if a predator has been leashed from the mind’s dark zoo. Those who endure them know: it can feel like a malevolent presence just off-camera in our bedroom.

Historical and Cultural Nightmare Demons

Historically and cross-culturally, such bad dreams have indeed been cast as external demons. Two centuries before Freud, a 1664 physician described a case of “Incubus or the Night-Mare,” where a sleeping woman believed a great dog or thief was choking her, exactly what we now know as sleep paralysis^[5]. The Old English word mare meant an evil spirit; Henry Fuseli’s famous 1781 painting The Nightmare immortalized this terror as a grotesque batlike incubus perched on a supine woman’s chest. This image has become shorthand for the squeeze of fear many experience in REM sleep^[6]. In fact, folklorists trace the nightmare demon across the globe: the East Indian Kanashibari, the Newfoundland “Old Hag,” even the Tanzanian bat-demon Popobawa, all lying heavy on the sleeper’s breast^[7][8]. Mothers in parts of the Middle East still protect infants with amulets, recalling Lilith, once a Sumerian night demon, and her progeny^[9]. Medieval Europeans prayed before bed to ward off such nocturnal visitors^[10], and Native Americans wove the now-ubiquitous dreamcatcher to trap “evil” dreams in its web^[11]. These rituals underscore a near-universal impulse: to externalize nightmares as enemies that can be locked out of the mind.

Defining Nightmares Clinically

Yet modern sleep science paints nightmares as intrinsic brain phenomena. Clinicians define a nightmare in strictly neurological terms: a REM-stage dream so frightening it rouses the sleeper^[4][12]. Unlike sleep terrors (which erupt from deep non-REM sleep with screaming but no dream-memory), nightmares typically occur in later sleep when REM episodes lengthen, often in the early morning hours, and the vivid dream narrative is still being woven^[13][14]. By definition, nightmare episodes leave the dreamer alert and oriented on waking, albeit shaken. And unlike benign dreams, the emotional tone is overwhelmingly negative: anxiety, fear, terror (occasionally anger or disgust) dominate^[4]. In fact, the American Academy of Sleep Medicine notes that repeated nightmares can disrupt sleep and spill over into daytime distress^[12], impairing concentration and mood.

The Neuroscience of Nightmares

On one level, dreams (and nightmares) are just byproducts of REM sleep: a paradoxical state of high brain activity in a paralyzed body. During REM, limbic regions, especially the amygdala, our brain’s smoke-alarm for threats, flare, while the frontal “logic” centers quiet down. The result is a vivid emotional play. “Nightmares are often thought to be an intensified form of adaptive dreaming,” write neuroscience researchers Tore Nielsen and Ryan Levin^[15]. According to their “affective network dysfunction” model, a healthy dream might use bizarre imagery to rehearse dealing with fear (fitting exposures in a safe rehearsal space). A nightmare, by contrast, represents that process breaking down^[15]. In other words, the brain is trying to consolidate and extinguish fear memories during sleep, but in nightmares the fear-spike remains unattenuated. Siegel and others have pointed out that REM’s neurochemical milieu plays into this: levels of norepinephrine and serotonin plummet, releasing the amygdala from daytime restraint. When this limbic alarm is already primed (say, by trauma or stress), the sleeper can feel unnaturally under siege.

For more information on They Sold Nightmares check out the production’s official page.

Brain Imaging and Fear Networks

Neuroimaging confirms the picture of a misfiring fear network. In the lab, people who frequently recall nightmares exhibit abnormal patterns of brain activity. For instance, a resting-state fMRI study found heightened synchronization in the anterior cingulate and other limbic-frontal circuits among nightmare-prone subjects^[16][17]. “Nightmares reflect dysfunction in a limbic-prefrontal circuit,” the authors hypothesize^[16]. In practice, that often means an overactive amygdala and an underactive medial prefrontal cortex, the reverse of a well-controlled fear response. Strikingly, neuropsychological case studies show that people with damage to the amygdala actually have fewer nightmares, suggesting that this little almond-shaped structure is a potent nightmare generator^[16]. On the other hand, studies of REM sleep physiology in nightmare sufferers report subtle differences (e.g. more frequent micro-arousals), consistent with the brain being chronically tuned to threat.

Psychological Correlates

Psychologically, frequent nightmares correlate with just about every anxiety measure on the book. General population surveys find that nightmares happen more often when life is stressful^[18]. Children and teens, especially around ages 3-6, routinely experience a spate of nightmares, perhaps an evolutionary relic as well, since small bodies are far more vulnerable at night^[2]. Adults with high emotional reactivity or maladaptive coping styles tend to have worse nightmares too^[19]. Those gender and age differences are notable: women report more nightmares than men at all ages^[20], and elders have the fewest, possibly as REM sleep thins with age. Perhaps unsurprisingly, nightmares are a hallmark of post-traumatic stress. Surveys suggest roughly half of trauma survivors have nightmares of the event (and up to 80% in full-blown PTSD)^[21].

Trauma and PTSD

Combat veterans, sexual assault survivors or accident victims often literally relive the trauma in sleep, their minds stubbornly rewinding the same terrifying tape^[22]. 2009 research found 80% of PTSD sufferers endure frequent nightmares^[21], often exact replays of the horrors they faced. In such cases, imaging shows the amygdala and related fear circuitry on constant alert even by day; dreamtime only completes the circuit.

Don’t forget to check out “They Sold Nightmares” Behind The Screenplay page for more information on the writing inspiration and process. If you’re inclined, you can also purchase the screenplay too.

When Myth Meets Science

Image: Concept art from Hugo Lunny’s They Sold Nightmares, visualizing a popobawa-like demon beneath a city. In Lunny’s dark fantasy, nightmares are literal goods traded by underground figures, a vivid metaphor for the terror our own brains can peddle^[8].

This link between our myths and our biology is more than poetic. Lunny’s novel (and screenplay) explicitly draws on the same nightmare lore: he even invents a “Popobawa Kanashibari Guild” and describes a protagonist who sells dreams in subterranean tunnels^[23][8]. In other words, his story offers a fictional mirror to neuroscience: just as his characters conjure nightmares for profit, our brains seem to conjure them automatically under intense fear. One particularly telling quote from the behind-the-scenes notes: “You don’t awaken from a nightmare delivered by Abdelaziz because whoever paid… gave up something invaluable to punish the recipient.”^[8] In plain terms, the recipient’s fear can’t wake them, they are trapped by it. Science, of course, would frame this differently: the sleeper might simply have an overtaxed arousal system and thus remain “locked” in REM until the threat peaks.

The Phenomenology of Nightmare Content

To the phenomenologist, the person experiencing the dream, nightmares have a heft that defies simple explanation. Often they symbolically recast waking anxieties. Surveys of nightmare content find themes of falling, being chased, or facing monsters are frequent, tapping into primal fears. (One large study of “most recent nightmares” found the top topics were failure/helplessness, physical aggression, being chased, and death^[24], common threads in human anxiety.) When asked to interpret dreams, people might recall loved ones, childhood scenes or personal traumas; nightmares sometimes invert these memories, presenting egregious betrayals or horrors. Anecdotally, nightmare themes can escalate in series: some therapists recount cases where a dreamer sees the same dark figure night after night, gradually closing in. Harvard psychologist Patrick McNamara notes that in serial nightmares, demons in dreams advance each time toward the dreamer’s bed, as if rehearsing an assault^[25]. Such patterns reinforce the intuition that nightmares “mean” something, even if science remains agnostic.

Clinical Treatment Approaches

Clinically, nightmares themselves do not always require treatment, occasional nightmares are part of normal dreaming^[12]. However, chronic nightmare disorder can severely affect sleep quality. Sleep experts view nightmares as a form of emotional dysregulation during sleep^[26]. Therapies have sprung up. Cognitive-behavioral techniques like imagery rehearsal therapy ask patients to re-script the ending of their nightmare and rehearse it while awake, reducing recurrence^[27]. Another approach is medication: for example, the drug prazosin (originally an antihypertensive) has been shown to calm nightmares in PTSD by dialing down hyperactive neurochemicals^[28]. These practical treatments reflect the scientific stance: nightmares are not messages from beyond but malfunctions of neural processes, inconvenient, yes, but not cosmic.

The older narratives aren’t entirely vanquished. Some people still interpret nightmares spiritually or psychologically (the “gateways to the unconscious” idea persists in popular culture). Sleep paralyzers might believe an Old Hag perched on them, and PTSD sufferers often feel cursed by fate. It is telling, though, that the most vivid myth of the nightmare is dovetailing with science. Consider Fuseli’s incubus one more time: his painting was essentially a clinical report of sleep paralysis^[6]. In that sense, art anticipated the neurobiology. Today, researchers can point to the exact network that painting glimpsed: an alert “fight-or-flight” brain stem nerve firing on top of REM atonia. They can measure it with EEG and imaging, even if it feels ineffable to the sleeper.

Sleep is still a frontier. Every night, our brains process emotions, cement memories, and test out scenarios while we recharge. Nightmares are a vivid reminder of how our neural machinery isn’t infallible. Far from being trivial, they implicate the same limbic engines that help us survive by staying afraid. Or, as Lunny’s darkness metaphorically suggests, they are one more thing we “sell” ourselves, unwanted goods from the market of the mind. The anxiety and mystery may endure, but understanding is growing. From ancient dream temples to modern MRI scanners, humanity has always sought to fathom the dark side of sleep. In the end, whether your mind calls it an incubus or a neural glitch, the nightmare is a signal: our inner world is alive even when we’re asleep, and sometimes it aches to be heard.

Broken English stuff:

What Are Nightmares Made Of? What are the origins of sleep paralysis in folklore? Behind The Screenplay – They Sold Nightmares