According to the psychology literature and memory research, the term memory consolidation indicates the gradual stabilization of long-term memory after acquisition, and within the time that memory phase, this stabilization takes place.
Before examining the link between consolidation and sleep, the term must be clarified. The term consolidation has evolved and is currently used to explain two distinct types of processes. “Synaptic consolidation” is used to describe cases in which only a short time has passed—typically minutes—after receiving information. On the other hand, “system consolidation” does not occur after minutes and involves a local consolidation. As the name itself suggests, it happens in the system and lasts weeks or even longer (Dudai, 2004).
The relationship between sleep and memory has been revealed in many articles with different focus points. The state of sleep has been considered as an optimization process of memory consolidation in recent findings.
Quintilian, the author in the history of classical rhetoric, is the one who talked about consolidation in the first place and explained sleep as a way to strengthen memory consolidation. To him, even a night can play a huge role in shaping recollection (Quintilian, first century A.D.).
In language learning, several complex aspects of language need to be consolidated and integrated into long-term memory. Aspects such as the intonations and pronunciation of the words, the grammatical rules of the language, and many other components. The brain necessitates a decent network system to access and recall the new organization of the words to become competent in that language. Overall, these components make language learning more than understanding concrete words and challenge the brain in the process of consolidation.
In part of the special issue called “Sleep and Language Learning,” the effects of sleeping on long-term memory have been investigated. 6.5-month-old infants were exposed to an artificial language for 7 minutes, and while one group from this sample slept, the other group did not. Based on the results of the experiment, those who remained awake showed no retention, whereas the nap group showed a notable difference (Simon et al., 2017). Furthermore, the information taken from Blume and colleagues’ research indicated that verbal information during both REM and Non-REM sleep stages was processed by the participants even though consciousness was minimal (Blume et al., 2017).
In the last study that this writing involves, the question of “Can we learn a new language while asleep?” can be seen more clearly.
According to the study of Blume and colleagues, our brain doesn’t always process information in the same way—it depends both on how alert we are and on how important or attention-grabbing the information is. The researchers looked at how people reacted to different types of sounds: their own names and angry voices (which are more attention-grabbing), compared to unfamiliar names and neutral voices (which are less attention-grabbing). These sounds were played while participants were awake, lightly asleep (N1), and in a deeper stage of sleep (N2). The findings showed that while people were awake, they paid more attention to emotional and personal sounds, especially angry voices. When people had just drifted into light sleep (N1), hearing their own name was more stimulating than hearing someone else’s name, no matter the tone of voice. Even in a deeper stage of sleep (N2), the brain still reacted more strongly to emotional or personally relevant sounds. This suggests that the brain keeps a kind of “night watch” during sleep, staying alert to important information. However, after this initial reaction in N2 sleep, the brain seemed to switch on a protective mechanism—like a built-in filter—that helped block further arousal and keep sleep from being disturbed (Blume et al., 2017).
Considering the brain activity patterns observed in this research, language learning and the ability to attend to novel words appear to be more effective during wakefulness. Without prior knowledge of the language and a period of study, acquiring an entirely new language (from scratch) is challenging—though perhaps not impossible. Furthermore, as the studies above suggest, consolidating newly learned words through sleep, following periods of learning and repetition, plays an important role in strengthening memory and supporting long-term retention.
References
Dudai, Y. (2004). The neurobiology of consolidations, or, how stable is the engram? Annual Review of Psychology, 55(1), 51–86. https://doi.org/10.1146/annurev.psych.55.090902.142050
Rasch, B., & Born, J. (2013). About sleep’s role in memory. Physiological Reviews, 93(2), 681–766. https://doi.org/10.1152/physrev.00032.2012
Simon, K. N., Werchan, D., Goldstein, M. R., Sweeney, L., Bootzin, R. R., Nadel, L., & Gómez, R. L. (2017). Sleep confers a benefit for retention of statistical language learning in 6.5-month-old infants. Brain and Language, 167, 3–12. https://doi.org/10.1016/j.bandl.2016.05.002
Blume, C., del Giudice, R., Lechinger, J., Wislowska, M., Heib, D. P. J., Hoedlmoser, K., & Schabus, M. (2017). Preferential processing of emotionally and self-relevant stimuli persists in unconscious N2 sleep. Brain and Language, 167, 72–82. https://doi.org/10.1016/j.bandl.2016.02.004