What horrors lurk in our subconscious? According to a new French study – believed to be the largest into the subject – on sleep-talking and what we say when we’re asleep, researchers found the most commonly used word is “no”, and the French swearword “putain” occurred, reports the Times, “800 times more often in sleep than when awake”. Some people were verbally abusive. The study also found that men sleep-talked more than women and used more profanities.

Sleep-talking, or somniloquy, is one of a number of a parasomnias (which include sleepwalking and night terrors). “In my experience, [it] can be linked to the nervous system being overstimulated, which can be related to excessive use of technology before bed or too much caffeine,” says Dr Nerina Ramlakhan, sleep therapist and author of Fast Asleep, Wide Awake. “But these sorts of behaviours can happen with people who are quite hard on themselves, they’re perfectionists, but they often hold back on saying what they really want to say. When they go to bed at night, it spills out into the sleep.”

The idea that you can unearth someone’s deepest secrets while they are asleep is compelling. There are apps that record sleep-talking (Sleep Talk Recorder allows people to upload their often hilarious nocturnal mumblings – one woman says: “Butthole, what was that”; another whispers: “Judi Dench”). But can you confess to a murder in your sleep? Or to having a lover?

Prof Jim Horne, former director of Loughborough University’s sleep research centre and author of Sleeplessness, suggests we shouldn’t read too much into what is said. “Sleep-talking tends to occur in very light sleep,” he says. “You go from a light to a deep sleep, and after about 70-90 minutes, you go into a period of dreaming sleep, which re-occurs about every 90 minutes.” Sleep-talking isn’t related to dreaming but to the lighter sleep where “the mind is meandering and ruminating. It’s really the ramblings of a rather befuddled mind.”

It is much more common in children, and the majority of people grow out of it, although it could have a genetic factor. “Anxiety and stress tends to bring it on,” says Horne. “[Sleep-talkers are] generally reflecting some sort of aspect of a worry. I think it’s best not to place too much concern on sleep-talking and not take the words said by someone seriously.”

However, sleep-talking might tell you one thing. “Someone who persistently does it is probably quite anxious and it might be a good idea to find out what is worrying them.”