My students have asked many times in which language I dream. They smile when I answer that all I can remember is how well I can, in my dreams, speak any of the languages I have learnt and the disappointment when I wake up.

It reminds me of a program I used in workshops to demonstrate the mental effort required to learn a second language in adulthood, even in old age, when your brain is monolingual. However, the benefits are equally powerful.

We all know that speaking several languages has advantages for communication. But it can have other benefits too. Could we be better at multitasking or have less risk of suffering from dementia? Scientists have debated these issues for over a decade without fully agreeing. They do know that using several languages changes our brains physically and how they work. You don't have to be bilingual from birth to leave a positive imprint on your brain. Let's see why.

The brains of bilinguals are physically different from those of monolingual people: there are certain language-related parts of the brain that are more robust or have changed in their way of connecting with others. These physical changes are a consequence of the repetitive use of the tools that our brain constantly uses to interpret everything that surrounds us, which are called cognitive processes.

The primary four are perception, memory, attention and emotion. As bilingual, these cognitive processes are exercised more, leaving physical marks in our brains. It is important to clarify that these differences are minor. Neither means that a bilingual's brain is generally better than a monolingual's. Even the most sceptical of the supposed advantages of bilingualism accept that there are three cognitive domains in which bilinguals tend to perform better.

Bilinguals have a greater capacity for operating memory. Since they face contextual, conversational situations that are a little tougher than monolingual people, they develop this working memory a little better, with which they can keep more elements in. And also, the work they do with these elements is significantly better than that done by monolinguals.

Bilinguals show a greater capacity to take another person's perspective. We are not talking about empathy but a more situational capacity to see reality from other angles. They have to be contextually very agile social beings to know what is the dominant language at the moment, what is the language in which the other person wants to communicate, and what is the language that I have in my repertoire that should use at this time. It is called the perspective-taking process, and multilingual people have been shown, from children to older adults, to be capable of better perspective-taking than monolingual people.

There is a nuance to all this: these functional differences observed in the laboratory do not necessarily translate into advantages for general life or work.

Finally, the bilingual brain shows a greater cognitive reserve. It tends to age better because it can more than compensate for the deterioration that occurs naturally as we get older. Interestingly, scientists are also observing that bilingual people cope better with neurodegenerative diseases, such as Alzheimer's or Parkinson's.

If the disease comes to you, it will come to you either a little later or its symptoms may be a little less. This is mainly due to the increase in the cognitive reserve of bilingual people compared to monolinguals. In the case of dementia, for example, the results show an average difference of four and a half years in the appearance of the first symptoms.

Disadvantages? I would call them micro-disadvantages of being bilingual. First, bilinguals can take milliseconds longer than monolinguals to find the word they seek. The second is that, in general terms, bilinguals tend to have a slightly smaller vocabulary in their two languages. But what is clear is that there is no cognitive disadvantage to speaking several languages.

An interesting fact to finish? Bilingual people who use sign language are the only people who can speak two different languages simultaneously.