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Somniloquy is very fascinating.

It is unclear what are the causes, but most of the time it is not seen as a medical condition.

"Sleep talking usually occurs by itself and is most often harmless. However, in some cases, it might be a sign of a more serious sleep disorder or health condition."

http://www.webmd.com/sleep-disorders/excessive-sleepiness-10/talking-in-your-sleep

Here is a brief descriptive testimony of someone having this condition:

This is what happened to me for about one year: almost every night, I talk unconsciously while sleeping (of course I don't know, but my room mate told me that). I dream almost every night and I consciously perceive that I'm dreaming, that I'm talking to someone, that what is happening in my dream.

Why does this happen? Is it a kind of mental disorder? Is it because my brain is too active and I keep thinking too much about one thing, so that it keeps obsessing me, even in my dream?

In other words, I understand that Bringing dreams to conscious awareness is possible, but why are facial muscles sometimes activated when talking in dreams? Are there correlations with other types of sleep disorders?

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Brief periods of sleep-talking (somniloquy), mostly gibberish, are quite common, particularly in children. It is not typically useful to call something a "disorder" unless it is causing a problem. Aside from disturbing others, sleep-talking is rarely problematic by itself, but in combination with other parasomnias (behaviours that indicate arousal during sleep) may be symptomatic of a more concerning sleep disorder.

Sleep-talking while dreaming is most often associated with REM sleep. However, note that not all sleep-talking happens while dreaming, and both dreaming and sleep-talking may also occur during transitions from non-REM sleep to waking state (partial arousal).

REM sleep normally involves a loss of voluntary muscle movement, referred to as REM sleep atonia, that prevents dreamers from physically interacting with their dreams:

When the body shifts into REM sleep, motor neurons throughout the body undergo a process called hyperpolarization: their already-negative membrane potential decreases by another 2–10 millivolts, thereby raising the threshold which a stimulus must overcome to excite them. Muscle inhibition may result from unavailability of monoamine neurotransmitters (restraining the abundance of acetylcholine in the brainstem) and perhaps from mechanisms used in waking muscle inhibition.

The process involved with REM sleep motor inhibition is complex and not fully understood, but presumably, a deviation from or malfunction in this process may result in temporary partial loss of atonia, or partial arousal during non-REM sleep, that can trigger sleep-talking. Causes of such anomalies include medication, alcohol and drugs, illness, genetics, mental disorders, and stress. Of course, partial arousal can also be triggered by external stimuli.

When associated with more complete loss of atonia, and occurring nightly or even multiple times a night, sleep-talking while dreaming is a common symptom of RBD, a sleep disorder in which patients act out their dreams. As RBD is itself potentially a symptom of a degenerative neurological disorder, significant chronic loss of atonia, including vivid sleep-talking, particularly in the elderly, would be more concerning than sleep-talking on its own.

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