We all know one person.

A sibling, best friend or other half - everyone has had to put up with someone who talks in their sleep at some stage or another, or maybe you're guilty of it yourself. One of my own sisters was serious sleep-talker as a child, meaning we'd just be drifting off at night when all of a sudden we'd be startled by a Father Jack-style outburst of profanities from across the landing. Interestingly, she's not alone. New research has shown that curse words are fairly common in sleep talk. Researchers conducted a study of 232 adults, including 87 people who experienced sleepwalking or night terrors and one with sleep apnea, at Pitié-Salpêtrière Hospital in Paris. The team monitored the participants sleeping over two nights and recorded 883 incidences of sleep talk, including 3,349 understandable words, reports Medical News Today. Of these words, there was one used by sleep-talkers more than any other - 'No'. In fact, almost a quarter of sleep talk was found to be "negative talk". The study also showed that 22 per cent of the sleep talk was made up of what the researchers deemed "nasty" language. They concluded that the reason for all the negativity and swearing is because sleep talk occurs in REM sleep, during which we have more intense, emotional dreams where we might be having an argument or feel like we're in danger, for example. Men tend to sleep talk more than women and swear more than women do in their sleep too. But what causes sleep talk, or somniloquy, to give it its medical name? The US National Sleep Foundation says it can be brought on by stress, depression, sleep deprivation, day-time drowsiness, alcohol and fever and that it can also run in families. Talking in your sleep could also be a symptom of another sleep disorder, like sleep apnea or night terrors, according to Arizona's Valley Sleep Centre.