You can learn foreign words as you sleep but it won’t make you fluent

Listening to foreign recordings and their translations while you sleep could help you learn a new language, although the extent of the benefit appears modest.

Knowing that our brains are always active, Flavio Schmidig, previously at the University of Bern in Switzerland, and his colleagues wanted to see if they could exploit this to help people memorise foreign words while they slept.

First, they monitored the brain activity of 30 German speakers while they spent one night in a sleep lab. The researchers focused on the first cycle of deep sleep, which lasts about 2 hours, says Schmidig, now at Tel Aviv University, Israel. Previous studies suggest we can process and store new information during deep sleep, the first cycle of which is characterised by waves of brain activity made up of peaks and troughs, each lasting around half a second, says Schmidig. Using data from a previous study, the team devised an algorithm that predicted when each participant was likely to have one of these peaks or troughs.

Next, half of the participants were played audio clips of words during these peaks, while the other half were played them during their troughs. Other research groups have previously carried out similar studies but were unsuccessful, possibly because they tried to teach people relatively long words without targeting them to specific points of their deep sleep. To overcome this, Schmidig and his team played two-syllable words – such as vogel, the German word for bird – that fit into the half-second cycles of peaks and troughs.

In their left ears, the participants were played 27 German words belonging to three categories: animals, places and tools. Simultaneously, a made-up word was played into their right ears, acting as a fictional translation. These fabricated words were used to overcome any prior knowledge of languages that the participants may have had, for example of French or English. Each word and its invented translation were played to the participants four times.

Twelve hours later, when they were now awake, the participants were played the made-up words and saw them written down. They were then asked which of the three categories the words belonged to. If they were simply guessing the answers, the researchers would have expected a success rate of 33 per cent. Those who had the audio clips targeted to the peak moments of their deep sleep answered no better than chance. Those whose clips were played during troughs in their deep sleep answered correctly 37 per cent of the time, but a statistical analysis suggests this could still have been the result of chance.

However, when both groups were tested again 24 hours later, those in the trough group answered correctly 41 per cent of the time, with a statistical analysis suggesting this wasn’t a chance finding. The peak group still scored no greater than chance.

The researchers aren’t sure why the participants only scored higher than chance when the audio clips were targeted at the troughs of their deep sleep. Nevertheless, this could one day be used to assist people who are learning a language, says Schmidig.

Gareth Gaskell at the University of York, UK, says the study was small and the effect size modest. “That said, the results do contribute to a growing understanding of what is possible in terms of learning in sleep,” he says. “What is interesting here is that the method exploits the natural cycles in sleep and targets the troughs as very brief periods of neural rest where new encoding may be possible.”

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