Right after I started working on this text, I had a very appropriate dream.
I hosted a party in a hotel suite, and the guests were from the United States (USA), Pakistan and other countries.
Most of the guests chatted in English, and one or two used German, my native language.
At one point I couldn't find my son and I panicked.
When I saw him, I sighed with relief: "Ach, if you were me!" (Aa, there you are!"), in German - and I hugged him.
If you speak more than one language, you may have had similar experiences of them interfering with your dreams.
In my dreams I often hear English, which I speak in my everyday life here in London, as well as German, the language of my childhood.
How and why does our brain create these multilingual dreams and can they affect our language skills in real life?
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Deciphering languages that appear in dreams
At first glance, it may not be surprising that many polyglots who use different languages during the day, and even people who are just starting to learn a foreign language, dream in those languages.
After all, the language we speak during the day mostly carries over into our night 'mode'.
Research on deaf and hard of hearing people has found that they use sign language in their dreams as well as when they are awake.
However, a more detailed analysis of multilingual dreams reveals that the picture is more complex.
For starters, instead of randomly replaying snippets of conversation throughout the day, our brains seem to mix them up with all kinds of worries, memories, and problems.
It can even 'write out' entire dialogues in an unknown, fictional language, or one that the dreamer heard that day but does not speak (in my dreams I sometimes have dynamic conversations in Japanese, which I studied but never really learned ).
Many of us seem to categorize the languages that appear in our dreams in certain ways - by person, place or period of life.
For example, people may speak in their dreams the languages they would speak in real life, while dreams about childhood are usually in the language of childhood - although one should be cautious about assuming that there is a common pattern, as only a few small studies have been conducted on dreams in multiple languages .
In addition, languages in dreams can have a cultural and national dimension.
An example is an American woman from Thailand who dreamed that she was buying a dress for her late sister, and she talked to her cousins about the choice in Thai and English.
There are also dreams in which language-related anxiety occurs - a person speaks a foreign language and tries to be understood by others, has to catch a train or plane from one language environment to another, or looks up words in a dream dictionary.
A research participant from Poland said that she dreamed of an English word that she did not understand - "haphazard" (disorganized) - and when she woke up she looked it up in the dictionary.
A participant from Croatia dreamed that he was trying and failing to communicate with a foreigner in Italian, German and English before realizing that they both spoke Polish, and then he felt relieved and laughed.
Researchers say that it is difficult to establish the exact mechanism and function of such dreams, partly because dreams are still a rather mysterious phenomenon.
What is much better understood is how and why our brain processes languages and even learns new words while we sleep.
At least that clears up the conundrum of dreaming in multiple languages.
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Processing words in sleep
To understand the connection between sleep and language, let's start with one: your native language.
You may think that you mastered your mother tongue a long time ago, but in fact you are still constantly improving it.
Even adults learn a new word of their mother tongue every two days.
"As we grow up, we learn a lot of new words, especially during the first 10 years.
"But we do it all the time and we just don't notice," says Gareth Gaskell, a professor of psychology who runs the Sleep, Language and Memory Laboratory at the University of York in England.
When we learn a new word, we keep adding to our knowledge of it until we fully understand its use, says Gaskell.
As an example, he cites "breakfast" (breakfast), a word that most of us confidently use.
But when a new word comes along that sounds similar, it can rekindle uncertainty about that already existing word.
"In the last five years, you've probably heard the word "Brexit" [Brexit - the exit of the United Kingdom from the European Union], which sounds similar to "breakfast" (breakfast)," he says.
When the new word "Brexit" was often used alongside the existing word "breakfast", it would confuse people.
In order to use a new word correctly and distinguish it from other words that sound similar, we need to connect it to our existing knowledge, says Gaskell.
"And to do that, you need to get some sleep."
The integration of old and new knowledge happens precisely during sleep.
During the day, our hippocampus (the main part of the brain), which is responsible for receiving information quickly, absorbs new words.
At night, it transfers that new information to other parts of the brain, where it can be stored and connected with other relevant information.
This helps us to choose the right word in every situation, and suppress other similar words.
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A 'vocabulary in mind' marking system
This process is essentially the same regardless of whether the word is in a native or foreign language, says Gaskell.
In multilingual people, foreign words (foreign words) are also stored in a huge mental list and are similarly selected or suppressed.
"Imagine having some sort of tagging system in your memory," says Gaskell.
"So if you have a dictionary of German and English in mind, every word you know is labeled under a specific language, and when you speak, you suppress half of those words and focus on the other half that you need."
Did I do that when I spoke to my son in German in my dream in a hotel suite where there were people who spoke English and German, picking words from my 'language store' and giving them meaningful meaning?
That would be a nice explanation, but unfortunately, the process of integration and consolidation occurs during a stage known as deep sleep or slow wave sleep.
Complex dreams, such as my hotel dream, usually occur during a phase known as rapid eye movement (REM).
"Some people argue that REM has a role in this whole consolidation process, which is to sort things out and maybe make some repairs," says Gaskell.
Commenting on my dream, during which I at one point ran away from the party to attend an online meeting of the BBC team, Gaskell says: "It's a really classic situation where some of your recent memories are connected to much longer-term knowledge."
"It fits really nicely into that story [that dreams help consolidate memories]. But right now, it's only at the level of conjecture."
What we do know is that in addition to processing daily information, our brains can also learn new words while we sleep.
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Mark Sust is a research group leader at the University Hospital for Psychiatry and Geriatric Psychotherapy in the Swiss city of Bern, which specializes in the neuroscience of aging, sleep and memory.
He and his collaborators invented pseudo-words, such as “tofer", and matched each with a German word such as "Tree" (baum - tree).
Each participant was told a different meaning so that the pairing would be random and without any accidental sound associations.
Then they played the word pairs to the participants while they slept.
The next morning they were asked if the "topher" would fit in the shoe box.
This isocol question confirmed a known limitation of learning new information during sleep: we generally cannot use that information in a conscious, accurate way when we are awake.
"The participants could not consciously reproduce that knowledge and say, 'tofer clearly means wood,'" says Sust.
"It was more their intuition that it was a big or small object".
About 60 percent of the participants gave the correct answer that the "tofer" would not fit in the shoe box.
Crucially, both words - "tofer" and the German word - had to be played to the participants during slow-wave sleep, specifically during the peak of slow brain waves.
When the researchers missed that peak, the pairing was not learned.
Mathew Koroma, a postdoctoral researcher at the University of Liège in Belgium who specializes in sleep and cognition, has co-authored a number of studies that shed light on the complex question of how and when language occurs in sleep.
"Basically, the message is that you can learn [words in other] languages while you sleep, even new languages you've never heard before, but you do it in a very different way than when you're awake," he says.
He and his team discovered that during sleep we can distinguish fake from real language.
During sleep, the participants were simultaneously played two recordings - real speech in their native language in one ear and nonsensical, pseudospeech in the other ear.
During that time, the researchers recorded their brain activity using the electroencephalography (EEG) method.
The EEG results showed that the participants' brains focused on real speech during sleep, but not on fake speech.
However, during the REM stage of deep sleep, participants mostly blocked or suppressed incoming speech.
Koroma suggests that a possible reason is that the brain is focused on internal processes: "When we are in a deep phase of sleep, we block out things that can disrupt our dreams."
In another study conducted by the same team, participants were played Japanese words in their sleep alongside sounds that hinted at their meaning.
For example, the word 'inu' (dog) was played along with the sound of barking, and the word 'kane' (bell) was accompanied by the sound of a bell.
Different words were played during two different stages of sleep: light sleep and deep REM sleep.
In this case too, the researchers recorded the brain activity of the participants using the EEG method.
When awake, participants were able to correctly associate the words they heard during light sleep with the corresponding pictures.
For example, the accuracy of matching "inu" with a picture of a dog was higher than the probability of a random outcome.
However, regarding the words played to the participants during the REM phase, the outcome was different.
"Whenever we've looked at REM sleep, the stage where we have the most intense sleep, we haven't been able to find solid evidence that a learning process has occurred," says Koroma.
He clarifies that this doesn't mean we can't learn during that phase, but that more research is needed to understand whether this is possible.
Encouraging learning throughout the day
Does all this mean that we can effortlessly learn Japanese in our sleep if we listen to the lessons all night to be sure to catch the right stage of sleep?
Not necessarily. It could actually be counterproductive because it can interfere with rest, says Koroma.
He points out that during the research, the participants learned words much faster when they were awake than when they were asleep.
"Learning is much more effective when you're awake."
And they could use the words much more confidently, because they had learned them consciously.
"Being awake is good for learning, and sleeping is more for repetition, not learning a new language," explains Koroma.
"It's an interactive process that complements each other, which means that during the day you're learning, and while you're sleeping you're sorting through that information, consolidating some of your memories, and trying to put them into new contexts."
Are there other ways we can use sleep to learn a language?
"Probably the best way is to learn a new language before you go to bed, and while you sleep play some of the words you just heard," says Koroma.
"The best results are achieved if they are played quietly enough, as this will increase your learning abilities.
"But if you play them too loud, it actually reduces your ability to learn." So fine-tuning is needed."
Sust from the University of Bern advises to learn new words during the day and rest at night.
"That way the brain will do what it needs to do."
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Solving problems in sleep
Researchers are cautious about the possible role of multilingual dreams in the process of learning in sleep.
"It's very, very difficult to determine how dreams in multiple languages could fit into that," says Sust.
Partly because the broader cognitive purpose of dreams is still unclear.
One possibility is that it's more of a byproduct of "brain activity and sorting through bits of memory," says Sust.
This does not mean that dreams are not connected to the language learning process at all, just that they are probably a consequence, not the main event.
"It is entirely possible that during multilingual sleep the brain is trying to connect the two languages," says Sust.
But due to the chaotic, individual nature of dreams and different native languages, it is difficult to say anything more specific.
Koroma points out that the REM phase is associated with problem solving and emotion regulation.
Similarly, dreams can allow us to use new words or phrases in different situations, or to explore feelings in the languages we speak, he explains.
A similar conclusion was reached by Danuta Gabris-Barker, professor of psycholinguistics at Silesia University in Poland, in an analysis of the dreams of polyglots.
He indicates that such dreams can express "fears and desires" regarding learning a foreign language, among other things, the desire to speak that language completely correctly, like one's mother tongue.
That idea would fit nicely with research that suggests wrestling with words or tasks in our dreams can be helpful in real life, for example, for wordplay, problem solving, or dealing with emotions.
But, as emphasized by Koroma and others, this is a possibility, not a proven fact.
So my multilingual dreams are a bit of a mystery in that case, at least in terms of their practical function.
However, learning about my brain's overnight acrobatics made me almost in awe of the hidden effort it takes to learn even a single word.
And while I was writing this article, I learned a new foreign word, although not while I was dreaming.
It is hypnopédie (hypnopedes), French word for sleep study.
I learned it from Koroma, a researcher in Belgium, who uses it in one of his articles.
It's been a few nights since I found out about her.
I wonder how my nocturnal brain labeled that word and what it associated it with - French, Belgian, dream, and a text delivery deadline, perhaps?
Hmm, it could be the start of some new interesting dream.
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*Sophie Hardah is the author of the book Languages are good for us (Languages Are Good For Us), which talks about the weird and wonderful ways people have used languages throughout history.
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