Though it's somewhat mysterious, sleep talking (a.k.a. somniloquy) is a common phenomenon. About half of kids talk in their sleep at least once a year (and less than 10% do it every day). And roughly 67% of adults talk in their sleep at least once every three months. It often runs in families and tends to be more common among men.
Sleep talking can vary considerably in its content—ranging from gibberish or mumbling to nonsensical outbursts to full, coherent sentences. Sometimes it may seem as though the sleep-talker is giving a speech or monologue, while other times it can sound as though the person is carrying on a conversation with someone else. The chatter can occur during any stage of sleep.
If a friend or family member has ever told you that you talk in your sleep, your first concern might be whether you said something offensive (such as a curse word) or revealing (like a long-held secret). Your second thought might be whether sleep talking is something to worry about, as far as your health is concerned.
This kind of nocturnal narrative falls into the "parasomnia" category, so it is considered a sleep disorder. In terms of what causes it, insufficient sleep, alcohol or drug use, illness, stress, anxiety, and depression are common triggers for these unconscious musings. Sometimes this nighttime chatter accompanies other sleep disorders such as sleepwalking, night terrors, sleep apnea, and REM behavior disorder. In rare situations, somniloquy is associated with a psychiatric disorder or nocturnal seizures.
If you talk in your sleep only occasionally and you are not experiencing symptoms of any of the other sleep disorders or health conditions mentioned above, then it's probably nothing to worry about. But if, for example, you're frequently waking up tired, or you feel overwhelmed or sad most of the time, it's worth talking to your doctor about sleep talking to make sure that it isn't a red flag for another health problem.