Most people think of sleep as a passive state — you close your eyes, you’re out, you wake up. But the brain doesn’t simply switch off. It cycles through complex stages of activity, and sometimes things go wrong at the boundaries between wakefulness and sleep. The result is parasomnias: abnormal behaviors, movements, or perceptions that occur during sleep or during transitions in and out of it.

If you’ve ever watched someone sleepwalk or scream in apparent terror with no memory of it the next morning, you’ve witnessed a parasomnia.

NREM Parasomnias: Disorders of Partial Arousal

The most common parasomnias occur during non-REM sleep, typically during the deep slow-wave sleep that dominates the first third of the night. They’re called “disorders of partial arousal” because the person is caught between deep sleep and wakefulness — parts of the brain are awake enough to generate complex behavior, while other parts remain asleep.

Sleepwalking (Somnambulism)

Sleepwalking affects about 4% of adults and is more common in children (up to 17% sleepwalk at some point). Episodes range from sitting up in bed to walking through the house, opening doors, or even leaving the building.

Sleepwalkers’ eyes are typically open but glazed. They can navigate around obstacles but their judgment is impaired, which is why injuries are a real concern. The person almost never remembers the episode. If you wake them mid-episode (which isn’t dangerous, despite the popular myth), they’ll be confused but not harmed.

Confusional Arousals

These are milder than sleepwalking. The person partially wakes up, often appearing dazed and disoriented, and may speak incoherently or behave in strange but non-violent ways. Episodes usually last a few minutes and are forgotten by morning. They’re especially common in young children and typically outgrown.

Sleep Terrors (Night Terrors)

Sleep terrors are dramatic and frightening — for the observer, at least. The person suddenly sits up in bed, screams, appears terrified, and shows intense autonomic activation: rapid heart rate, dilated pupils, sweating, and fast breathing. They may thrash around or bolt out of bed.

Despite the intensity, sleep terrors occur during NREM sleep, and the person isn’t experiencing a dream or nightmare (those happen during REM). They’re genuinely not aware of what’s happening and typically have no memory of the event the next day.

Sleep terrors are most common between ages 3 and 12, but they do occur in adults. In children, they’re usually benign and self-limiting. In adults, they can be associated with stress, sleep deprivation, or alcohol use.

The key distinction from nightmares: nightmares happen during REM sleep, the person wakes up fully, and they can describe the dream content. Sleep terrors happen during NREM sleep, the person doesn’t fully wake, and they can’t describe any dream.

REM Parasomnias

REM Sleep Behavior Disorder (RBD)

This is the one that worries sleep physicians the most. During normal REM sleep, the brain actively paralyzes skeletal muscles — a process called REM atonia. This is why you don’t physically act out your dreams. In RBD, that paralysis fails.

People with RBD physically act out vivid, often violent dreams. They punch, kick, yell, and can injure themselves or their bed partners. Unlike NREM parasomnias, they often remember the dream content upon waking.

RBD overwhelmingly affects men over 50, and here’s the concerning part: a significant percentage of people with RBD eventually develop a neurodegenerative disorder — most commonly Parkinson’s disease or dementia with Lewy bodies. This makes RBD an important early biomarker, and it’s one reason clinicians take it seriously.

Nightmare Disorder

Everyone has nightmares occasionally. Nightmare disorder is diagnosed when frequent, disturbing dreams cause significant distress or impair daytime functioning. It’s particularly common in people with PTSD and can be treated effectively with image rehearsal therapy (IRT), a technique where patients mentally rehearse a modified, non-threatening version of the nightmare during waking hours.

When to See a Specialist

Most childhood parasomnias are outgrown and don’t need treatment beyond reassuring the parents and making the sleeping environment safe. But certain situations warrant a medical evaluation:

• Episodes that persist into adulthood or begin in adulthood for the first time
• Behaviors that risk injury to the person or others
• REM-related parasomnias (especially potential RBD) in older adults
• Parasomnias accompanied by excessive daytime sleepiness, which may indicate an underlying sleep disorder triggering the events
• Frequent or distressing nightmares, particularly if associated with trauma

A sleep study — specifically a video-augmented polysomnography — can capture episodes, confirm the diagnosis, and identify any contributing sleep disorders.

Living with Parasomnias

For many people, parasomnias are infrequent and more of a curiosity than a clinical problem. Consistent sleep schedules, adequate sleep duration, limited alcohol, and managed stress all reduce the likelihood of episodes. For those with more frequent or dangerous events, treatments range from environmental safety measures to medication. The first step is always the same: take it seriously enough to get it properly evaluated.