Can You Hear While Sleeping?

Hearing functions quite differently than the other senses. With the ability to blink or close one’s eyes, it is possible to shut out visual information when it is not useful, for instance during sleep. However, without the ability to blink our ears, they are constantly inundated with sonic information from birth to death. Despite the fact that these soundwaves enter the ears in a constant stream through the lifespan, our brains take over the function of “blinking” out sonic information that is not useful to us. This ability to filter out sound seems to apply to sleep as well as waking consciousness, but our filtering abilities work to varying degrees. A recent study engaged in a language experiment to test this ability of the mind to hear while sleeping. The results were surprising, and they inform us about the possibility of using the mind to focus the process of hearing. 

The Study

In order to better understand how the brain processes sound while sleeping, an international team of researchers in France and Denmark combined their skills. These scholars affiliated with École Normale Supérieure in Paris, the Technical University of Denmark, and the Sorbonne engaged in a study that presented waking and sleeping people with language-based tasks. The first task was to sort words into the categories of animals and objects, such as “cat” versus “hat.” The second task was to distinguish real words from made up ones, such as “hammer” versus “fabu.” The participants were simply asked to press a different button for the two categories, and they worked at the task long enough to get in the flow of the activity. The lights were turned off in the room, and they were in comfortable chairs. 

After developing comfort with the task, participants were encouraged to fall asleep when they wanted to. At this point, EEG readings on brain activity become an important way to monitor what the brain did with sound after falling asleep. In this light phase of sleep, the brain continued to activate the parts of the motor cortex that would prompt them to press buttons. Although they were no longer pressing buttons, the continued presence of words made their brains reach out as if to do so. The participants did not remember the words after they woke up, but their brains had operated as if they were continuing to process sound when it came. One fascinating extension of the study was to continue to monitor brain activity linked to the sound of language in deep REM sleep. Researchers were surprised to find that the brain did not activate the motor cortex in this mode of sleep. They had expected to see that type of brain activity continue, because the brain is quite active during REM sleep, almost displaying the same levels of activity as in waking life. On the contrary, the brain no longer seemed to be processing language in the same way. 

The Implications

This study lets us know quite a lot about the processing and filtering activities of the brain when it comes to sound. Audiologists had already come to understand the “cocktail party effect” in waking life. This effect makes it possible to use nothing but the focusing power of the brain to isolate one voice in a room full of speakers, even when their voices are at the same volume. Whereas the eyes use motion to train themselves on the objects of sight, the ears work in concert with the brain to make focused listening possible, even with other loud sounds in the environment.

This study points to a similar ability during sleep. You might wonder why it is possible to sleep through a loud thunderstorm and then to wake up to the sound of your relatively quiet alarm clock. The brain is not only sensitive to the volume of sound, but in many circumstances, it is also able to isolate important sounds of interest from the other background sounds that are less important. 

For these reasons, hearing loss does not only impair your ability to hear in waking life but also while sleeping. Should an emergency occur while sleeping, our ears rise to the responsibility of waking the brain from sleep, and hearing loss can pose danger in these situations. If you have not yet had a hearing test, the time is now to make an appointment and to seek treatment if necessary. Contact us today!