Sleepwalking is a common sleep disorder characterized by complex motor behaviors that occur while a person is still asleep. Episodes can range from simply sitting up in bed to performing intricate activities, all without conscious awareness. Understanding how the brain cycles through the different phases of sleep reveals the specific stage in which sleepwalking is rooted and why these events happen.
Defining the Phases of Sleep
Sleep is organized into distinct phases: Non-Rapid Eye Movement (NREM) and Rapid Eye Movement (REM) sleep. NREM sleep makes up about 75% of the night and is divided into three stages of progressively deeper rest. Stage N1 is the initial, lightest stage, a brief transition between wakefulness and sleep where the body begins to relax.
Following N1 is Stage N2, where heart rate and body temperature drop, and brain activity shows characteristic bursts called sleep spindles and K-complexes. Most of the night is spent in this stage, which is considered a period of stable sleep.
The final and deepest phase of NREM sleep is Stage N3, also known as slow-wave sleep. This stage is characterized by high-amplitude, low-frequency delta waves. It is difficult to wake someone during N3, and if arousal occurs, the person is often disoriented. REM sleep is the phase associated with vivid dreaming, where the brain is highly active but the body experiences temporary muscle paralysis.
Somnambulism and Deep Sleep (NREM 3)
Sleepwalking occurs during Non-Rapid Eye Movement (NREM) Stage 3, the deepest period of the sleep cycle. This phenomenon is classified as a disorder of arousal, resulting from an incomplete transition from deep sleep to full wakefulness. The body’s motor systems become active while the cognitive and awareness centers of the brain remain asleep.
The brain attempts to move out of the N3 stage, but a sudden trigger causes a partial awakening, leaving the individual in a state that is neither fully asleep nor fully awake. Since N3 lasts the longest during the first half of the night, sleepwalking episodes tend to happen within the first one to three hours after falling asleep. This timing contrasts with REM sleep, which becomes longer and more frequent toward morning.
During an episode, brain activity shows a mixture of delta waves, characteristic of deep sleep, and faster wave patterns associated with wakefulness. This physiological disconnect explains how complex physical actions can be performed without conscious control or memory formation. The motor cortex functions enough to allow movement, but the prefrontal cortex, responsible for judgment and memory, remains dormant.
Common Manifestations of Sleepwalking
Behaviors range from simple, repetitive movements to highly complex actions. A person may simply sit up in bed, look around with a blank expression, or begin mumbling incoherently. The eyes are typically open, but the individual is unresponsive to external stimuli or communication attempts.
More involved manifestations include getting out of bed and walking through the house, attempting to open doors, or performing routine activities like getting dressed or moving furniture. These actions are often clumsy because the brain is not fully supervising the movements. Episodes are usually brief, lasting from a few seconds to a few minutes, though they can occasionally persist longer.
A defining characteristic of sleepwalking is amnesia for the event upon waking. The person has no recollection of the nocturnal activity, often learning about the episode only from a witness. While complex and potentially risky actions such as leaving the house or driving are possible, they are infrequent, with most episodes involving relatively simple ambulation.
Contributing Factors and Triggers
The frequency of sleepwalking events is often increased by several contributing factors. One significant trigger is severe sleep deprivation, which causes the body to spend more time in the deep N3 stage, increasing the opportunity for a disordered arousal. High levels of psychological stress or anxiety can also destabilize the sleep cycle and make episodes more likely.
A strong genetic predisposition exists, resulting in a significantly higher risk for individuals who have a direct family member who sleepwalks. External factors can also provoke an episode, including the use of alcohol or specific sedative medications, which alter the stability of deep sleep. In children, fever and general illness are frequent triggers, as they cause increased arousals from sleep.
Other underlying sleep conditions, such as obstructive sleep apnea, can increase the likelihood of somnambulism by causing repeated awakenings. If episodes are frequent, involve dangerous behavior, or begin in adulthood, consulting a physician is appropriate. Safety measures, such as securing windows and doors, are recommended to prevent injury.