Almost all of us have heard, at some point in our lives, a phrase: “You were talking in your sleep at night”. This phrase can cause different emotions, from surprise to embarrassment and concern, but it always raises a question: “And what did I say?”
Usually the answer is: “Nothing interesting”. Studies have shown that most “sleep talk” is not notable thoughts, but just short, insipid, “scattered” words that last one or two seconds.
According to the National Sleep Foundation (NSF) in the U.S., talking in sleep can occur at any sleep stage. At the REM stage, when dreams occur, it is usually caused as a result of the dialogues we see in our dreams.
Practically that means that the mouth and vocal chords, normally inactive while we sleep, are transiently activated and express aloud the words uttered by one of the “protagonists” of our dream.
Talking in sleep may occur during the transitional stages, i.e. when we pass from one sleep stage to another.
In both cases, the speech occurs when something “invades” in our sleep, which makes us talk, but prevents us from saying something that makes sense to anyone who is listening.
It is difficult to estimate the frequency of sleep talking, because it happens when we sleep and if there is no one awake next to us, we may never learn about the ‘incident’.
However, studies have shown that more than half of children sporadically talk in their sleep, with frequency tend to decrease as they grow older. Chronic sleep talking in adults is considered a sleep disorder and may occur due to stress and other factors.
Moreover, since talking in sleep occurs due to transient overlapping of different stages of consciousness, it usually lasts very little – a few seconds, during which we express a few words.
However, there have been rare cases when a person could have a whole conversation while sleeping. Even these cases are mostly incoherent, but despite they may seem to make sense, they should not to be taken seriously into account. That is why, according to NSF, science and the law do not consider sleep talking a reliable product of the conscious or rational thought.