A few people sleep like a log, and need a bucket of water splashed to wake them up. The rest of us are not so lucky and our sleep gets disturbed even if someone calls out to us loudly. But there’s a good side to being able to hear sounds even while we are asleep. Studies suggest that hearing sounds helps us in retaining and strengthening our memories.

The connection between sleep and memory

On a typical day we come across many people, events and situations. But not all of them find a place in our memory. We retain only the stuff that is worth remembering. In the barrage of information we are exposed to, this selective retention serves us. For instance, the share price of a certain company may be of great importance to the company’s shareholder who may remember the fluctuations much more accurately than a non-shareholder. In other words, having decided that a piece of information is valuable, we ensure that we don’t forget it by rehearsing and repeating it. And this rehearsing continues even in our sleep.

As we go into deep sleep at night, our brain goes through all the events that were marked ‘important’ and replays them. This makes the memory of those events stronger and longer lasting. Likewise, memories of the events and information that were not replayed tend to be weaker making them more forgettable.

The effect of sound

In context of memories, two dimensions of sound—frequency and meaning—are relevant. Let’s take the sound of train hooting. Its shrillness [frequency] may be irritating to many. But to a person crossing on the railway tracks, it could mean the difference between life and death [meaning].

The frequency of sound helps the brain to remember more efficiently while we are asleep. In one experiment, participants were exposed to white noise—the kind of sound of a click or the buzz of a radio tuned to a non-existent station—at a frequency that was synchronised to the rhythmic oscillations of the brain during sleep. There was no meaning in the sound. But, because the frequency matched the brain, the study participants were able to remember word associations better than those who were not made to hear such sounds. Also, those who heard sounds that were not in sync with the brain, did not seem to remember any better. Though the sound had nothing to do with words, participants were able to remember the word associations better.

The meaning aspect of the sound also plays a significant role in memory. The key to remembering something well is its rehearsal during sleep. A sound can be a strong trigger, provoking its rehearsal. Of course, if we could not hear sound while sleeping, that would never be possible.

The cat and the kettle

This was demonstrated by an interesting study in which 12 participants were asked to memorise the locations of objects on the screen. 50 images were shown, one after the other. Along with the image, a related sound was played e.g. a meow with the image of a cat or a whistle with a kettle. Then they were asked to sleep. In their sleep, only 25 sounds were played softly even as they entered deep sleep—almost as if whispering the sound in their ears. When they awoke and were asked to remember the location of the objects, they could remember the locations of those objects whose sound was played in their sleep. Yet, when asked whether they heard anything when they were asleep, the participants replied in the negative. So, the sound had done its trick subtly. The rehearsal of memories occurred in the brain when the subjects were asleep, and as a result the memories of those objects were strengthened.

The next study took the idea even further. Same procedure but the research team promised money if the participants were able to remember the location. If the participant remembered the location of the cat, that would fetch more money than remembering the location of the kettle. So the ‘cat information’ was more valuable than the ‘kettle information’. As the participants were packed off to sleep, the sounds of the low-value objects was played. E.g. the kettle whistle was whispered in their ears but not the meow. The results were right on cue. The participants remembered the locations of high value objects. But they forgot the locations of low value objects—except the low value objects whose sounds were played in sleep.

The implications of this research could be far reaching—it can help people learn better and quicker. It could also help older people suffering from memory problems. Student taking courses that need them to memorise something could be given some auditory cues during the session, which they could replay softly later at night. For example, a guitar teacher may use this technique to reinforce the lessons taught during the day by asking the student to listen to a recorded version of tunes softly while sleeping.

With this emerging research, sound sleep now has a new meaning altogether.

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