Pop on a pair of headphones before bed and learn Italian in your dreams. Play Vietnamese radio while you cook and absorb vocabulary while chopping onions. Turn on an Arabic TV show without subtitles and let the colloquialisms wash over you. In no time, you’re guaranteed to be fluent.

Right?

Endless websites, podcasts and even YouTube playlists promise that you can learn a language subconsciously just by passively exposing yourself to it, without any effort. Sit back, relax and let your subconscious do the trick. It sounds almost too good to be true. So…is it?

What is subliminal language learning?

The concept of picking up a language without trying is called subliminal language learning; even if you’re not paying attention to anything being spoken, your brain will automatically remember commonly repeated words and form connections between those words and their definitions in your native tongue. At least, that’s the idea.

In theory, this should work. After all, babies pick up their mother tongue this way. They don’t cram flashcards or glue themselves to the latest language-learning app. They silently absorb the world around them. Sure, adults’ brains are a little less malleable, but they should still be able to pick up common phrases through sheer exposure.

Can you learn in your sleep?

The most famous example of subliminal language learning is hypnopaedic learning—that is, learning while sleeping. A quick search for products promising to make listeners bilingual turns up dozens of get-fluent-quick products in everything from Spanish to Tamil.

There is a precedent for achieving fluency while unconscious, as dozens of people have emerged from comas completely fluent in another language. In 2010, a thirteen-year-old Croatian girl awoke speaking German like a native. In 2014, a twenty-two-year-old Australian man woke up with a perfect command of Mandarin. While comatose, they weren’t doing vocabulary drills and diagramming grammar, so some sort of hypnopaedic learning must have occurred. However, those are extreme cases. Can sleep-learning take place over the course of a single night?

The Swiss National Science Foundation is convinced that it can.

In 2014, Swiss psychologists at the University of Fribourg tested this with sixty German-speaking students. At 10 p.m., all of the students were taught a list of Dutch words that they had never seen before. Afterward, half of the students were directed to go to sleep while a recording of the new vocabulary played, and the other half listened to the same recording again while awake. At 2 a.m., the psychologists woke everyone up and tested both groups. Those who absorbed the new vocabulary while sleeping did significantly better at recalling the words than those who attempted to learn solely while awake. This, according to the psychologists, proves that hypnopaedic learning is not just viable—it’s more effective than conscious learning!

Or not.

Sleep-learning has severe limitations.

“People can’t learn any new verbal information while they’re asleep,” says Anat Arzi, a neuroscientist and sleep-learning specialist at the Weizmann Institute of Science. “It’s too complex for the brain.”

Opponents of subliminal language learning qualify or even outright reject the results of the study, citing numerous alternative explanations for the psychologists’ findings. For example, all of the German students had heard the vocabulary words at least once before, so while sleeping, the participants weren’t truly taking in any new information. If anything, their brains were just mapping connections among the words that they learned while awake.

Furthermore, it’s possible that they didn’t learn while they were asleep—they learned because they were asleep. Students allowed to rest will almost always perform better than exhausted students who have been forced to listen to the same monotonous vocabulary recording drone on and on until 2 a.m. According to skeptics, that’s not groundbreaking science as much as it is common sense.

Wanting to test hypnopaedic learning for themselves, linguists at the language website MosaLingua ran their own study in 2016. They chose one hundred thirty-six participants as young as eighteen and as old as over sixty.

First, the participants took control tests: memorization tests on sets of foreign vocabulary words they’d studied before and on sets they hadn’t. Then, over two weeks, they listened to recordings of words and phrases during the early stages of sleep, when the brain supposedly has its highest sensitivity to external stimulation. Every day, participants alternated between hearing words they had already attempted to memorize and hearing words that were brand new, and they completed tests when they woke up.

MosaLingua compared each participants’ daily results with the results of their control tests. After listening to recordings of words they’d never seen before, seventy-two percent of participants experienced absolutely no change to their test scores. They retained none of the new vocabulary from the recording. MosaLingua’s conclusion? Hypnopaedic learning is not nearly as effective as its supporters claim it is, and learning new words while sleeping is nearly impossible.

Keyword: new. Learning new words while sleeping is nearly impossible.

Sleep-learning is a great tool to solidify familiar vocabulary.

After listening to recordings of words they’d previously seen, nearly seventy percent of MosaLingua participants improved their test scores. Fifty percent of those who improved their scores did so by over forty percent. These results complement the findings of the Swiss study, where participants learned the vocabulary prior to hearing the recordings at night.

Remember the American man who woke up fluent in Mandarin and the Croatian girl with a German silver tongue? Prior to becoming comatose, the man studied Mandarin in high school and the girl took a German course. They were by no means fluent before their comas, but they had previously been exposed to the vocabulary that now came so easily to them; the prolonged sleep just solidified what they’d already learned. Clearly, sleep is the key to strengthening the recall of familiar vocabulary.

Are other subliminal language learning strategies effective?

Although it’s the most talked about, sleep isn’t the only method of subliminal language learning; some people swear by the power of playing a language in the background while you go about your daily life. Long commute? Turn on a Turkish podcast. Lazy Sunday in bed? Cozy up to a Spanish telenovela. Doing chores? A little Swedish pop music should do the trick. By just hearing your target language, even if you’re not paying much attention, you’re bound to pick up something, right?

This is true—to a point. Hearing your target language may help you become more comfortable with the way it sounds, so that the first time you find yourself surrounded by native speakers, you won’t feel (as) overwhelmed. And when you finally attempt to speak it yourself, you might subconsciously imitate those shows or radio hosts, allowing you to achieve a slightly more natural-sounding accent.

However, this method can only go so far. That is, not far at all.

If you’re not learning actively, you’re not learning much.

Simply put, passively listening to a language won’t help you parse what’s being said. Until you sit down and actively listen to the words—preferably with a dictionary handy—you can’t comprehend the meaning, so you might as well be listening to gibberish. Yes, you’ll become familiar with the way the language sounds, but you won’t truly grasp how to employ the words you keep hearing.

On its own, passive listening won’t magically perfect your pronunciation either. Sure, you may be able to subconsciously imitate a certain cadence, as I mentioned above, but if you want to truly speak like a native, you have to do more than just listen. You have to learn, repeat and practice what you hear.

Passive listening also has the downside of making you feel like you’ve productively studied your target language…even if you haven’t. After listening to a Cantonese podcast for an hour, you might feel accomplished and even a little tired, believing that you must have improved because of the sheer amount of time that you “invested,” even if you’ve actually learned nothing at all. In all honesty, you might have picked up more with five minutes of focused flashcard practice than with an hour of unfocused passive listening.

Subliminal language learning will never be as effective as active language learning.

Subliminal language learning alone cannot make someone completely fluent, whether they’re wide awake or fast asleep. And, let’s be honest: That’s disappointing. After all, who wouldn’t want to pick up a second language while eating breakfast or taking a nap?

Fortunately, even though you can’t rely solely on subliminal language learning, you can still take away important lessons from it.

1. Play vocabulary when you sleep.

Just be sure to study the vocabulary on your own first. Both MosaLingua and the Swiss study proved that if you listen to familiar vocabulary playing while you sleep, you’ll retain it better. So review the words once or twice before popping on the headphones.

2. Be strategic about when you learn.

In the Swiss study, participants who studied at 10 p.m., right before bed, did significantly better than participants who stayed up cramming into the wee hours of the morning. Find the time that’s best for you. Maybe you work efficiently at night, when you’re free of distractions. Maybe you prefer to learn new vocabulary as soon as you wake up, so that you can keep it at the tip of your tongue all day. Run your own experiments.

3. Listen actively, not passively.

What’s the difference? To be frank, passive listening is easy. Active listening is hard.

Passive listening doesn’t require any attention. Active listening requires full attention. Passive listening lets the words just wash over you. Active listening makes you wrestle with every syllable. Passive listening won’t get you far, but active listening will do wonders for your comprehension.

Instead of passively listening to something in the background for an hour, take ten to fifteen minutes to actively listen, tuning in closely to what’s being said.

As you do this, grab a highlighter and a dictionary. Research unfamiliar idioms. Keep a journal of new vocabulary, and use each word in an original sentence.

Repeat everything you hear, and record yourself to polish your pronunciation.

If you’re listening to a song, learn the lyrics and belt them alongside the lead singer.

If you’re watching a show, translate it as you go along, and then hop on an online forum to discuss it with other fans.

You have to make an effort, but it will pay off!

Keep going.

Sadly, subliminal language learning doesn’t really exist, at least not to the extent that people hope it does. However, even though it’s impossible to become completely fluent in your sleep, you can still use sleep, coupled with other learning strategies, to strengthen your vocabulary recall.

Active learning is hard, but don’t give up. Be intentional with when and how you study, and you will master your target language in no time.