Watching someone talk in their sleep can be funny and sometimes even scary, but what's happening in the brain when this takes place?
To understand what causes sleep talking - somniloquy, to give it its formal name - we must first understand what happens to us when we fall asleep.
Once we nod off, we alternate in cycles between two types of sleep: lighter REM sleep and deeper non-REM. REM stands for rapid eye movement where your eyes move quickly while closed. Dreams typically happen during REM sleep.
A lot of sleep talking will happen during the REM dream phase, but it can also happen throughout the night at any sleep stage. The National Sleep Foundation says: "The lighter the sleep, the more intelligible the speech: people may have entire conversations, while in (deeper) stages, speech may be restricted to moans and gibberish."
It is also thought that transitory arousal - moving from one sleep stage to another - may cause sleep talking.
The root causes could be at a chemical level. As we fall asleep our brains produce two paralysing chemicals: glycine and GABA. These keep your vocal chords, mouth and muscles in a kind of paralysis, allowing you to rest.
Scientists think some people might slip through their own net of paralysing chemicals and end up voicing what's happening inside their head - called a motor breakthrough.
Sleep talking may also be brought on by stress, depression, fever, sleep deprivation, day-time drowsiness, alcohol, and fever.
The National Sleep Foundation adds: "In many instances sleep talking runs in families, although external factors seem to stimulate the behaviour. Sleep talking often occurs concurently with other sleep disorders such as nightmares, confusional arousals, sleep apnea, and REM sleep behaviour disorder."