Why does a sleepwalker have no memory of their nighttime wandering?

Sleepwalking occurs during deep slow-wave sleep, a stage when the brain is too weakly active to encode episodic memories. The person can get up, walk, and even talk, because motor areas remain partly active, but the circuits for memory and consciousness stay offline. On waking, no trace of the episode remains.

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Sleepwalking: What Science Says

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A ghost is said to haunt the eastern ramparts of the imperial court: a white figure dances there every night, spreading terror among the servants and concubines. When Jinshi tasks Maomao with solving the mystery, he asks her point-blank whether she knows anything about sleepwalking. She admits her specialty is plants, and that ailments of the mind fall outside her field. But it is Maomao’s father, a renowned physician, who supplies the definition: sleepwalking is an ailment of the mind that drives the patient to act as though awake. The scene also underscores a point modern medicine confirms: this kind of disorder can be faked, as one courtesan used exactly that ploy to lower her buyout price.

The subject in depth

Sleepwalking belongs to the family of parasomnias, disorders that occur during sleep without the person being aware of them ^[Wikipedia]. It is clearly distinct from REM-sleep parasomnias such as nightmares or sleep paralysis: it occurs during deep slow-wave sleep, also called stage N3, which dominates the first half of the night ^[Wikipedia].

Diagram of the sleep cycle highlighting stage N3 (deep slow-wave sleep) where sleepwalking occurs — One night of sleep, stage by stage

During this stage, slow brain waves signal a state of very low overall cortical activity. Yet certain motor regions remain active enough to produce elaborate behaviors: getting up, walking, opening doors, sometimes speaking. What the brain stops doing is processing sensory information coherently, maintaining self-awareness, and above all, encoding what happens into episodic memory. When the person wakes up, no trace of the episode remains. It isn’t that they forget; memory consolidation simply never took place.

A sleepwalker’s eyes are most often open, which feeds the confusion with a waking state. Their gaze is fixed, poorly reactive. They may answer simple questions, but the answers are incoherent. They avoid obstacles in a rudimentary way. Their behavior is stereotyped and lacks clear purpose, unlike that of someone genuinely awake.

Prevalence is significant: about 2 to 4% of adults have recurrent episodes, and up to 15% of children have experienced at least one, making it one of the most common parasomnias ^[Wikipedia]. Documented triggers include sleep deprivation, stress, fever, certain medications acting on the central nervous system, and genetic predisposition: the risk is markedly higher in children with a sleepwalking parent.

The question of simulation is medically serious. Without polysomnography, the simultaneous recording of brain waves, muscle tone, and eye movements during sleep, it is clinically difficult to distinguish a genuine episode from a careful imitation. Certain markers guide the differential diagnosis: a true sleepwalker shows characteristic slow waves on EEG, a particular muscle tone, and no response to complex verbal stimuli. But without a laboratory sleep study, these criteria remain out of reach of direct observation.

In real life

Long before neurology offered an explanation, sleepwalking was interpreted as possession, an omen, or a visiting spirit across many cultures. In medieval Europe, a person caught wandering at night could be suspected of witchcraft; in classical China, it was attributed to an imbalance of vital energy. Until the nineteenth century, prevailing theories linked sleepwalking to lunacy, the idea that the moon influenced the humors, despite the absence of any physiological basis. Modern medicine only clearly established the link with sleep stages from the 1950s and 1960s onward, thanks to the first polysomnographic recordings. Today, treatment is mainly preventive: improving sleep hygiene, reducing triggers, and securing the nighttime environment. Severe cases may benefit from medication, but this remains reserved for situations where episodes expose the person to real danger.

The fact-checker's eye

The definition given by Maomao’s father is medically sound in its broad strokes. Saying the patient acts as though awake without being awake captures the essence well: partial active motor behavior, absent consciousness, no memory. The episode also raises the idea that a convincing simulation is possible, which matches medical reality: without polysomnography, differential diagnosis is difficult, and forensic literature records legal cases where the question was seriously debated. The only shortcut in the story lies in the phrase “an ailment of the mind” itself: it’s a metaphor for the functional disorder, not a mystical cause. The series doesn’t claim otherwise; it uses the phrase as accessible language for a medieval setting, which is narratively honest.

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Published on May 30, 2026

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