Imagine studying for a test while the faint smell of roses fills the room. That night, after you have fallen asleep, someone introduces the same scent into your bedroom. Could the smell somehow help your brain remember what you studied earlier?
It sounds rather far-fetched, but researchers have actually tested something very similar — and what they discovered provides a fascinating glimpse into what our brains do with memories while we sleep.
The rose experiment
In a study published in Science in 2007, researchers led by Björn Rasch investigated whether a smell associated with learning could influence memory during sleep.
Participants had to learn the locations of pairs of pictures on a computer screen, rather like playing the familiar game of Memory. While they were learning, they were exposed to the scent of roses. Later that night, once they had fallen asleep and entered slow-wave sleep, the researchers exposed them to the same scent again.
When their memories were tested afterwards, participants remembered the picture locations better when the rose scent had been presented during slow-wave sleep. Brain imaging also showed increased activity in the hippocampus, a region of the brain that plays an important role in forming and retrieving memories.
The researchers had apparently found a way of using a smell to cue a memory while a person was asleep.
It wasn’t simply the smell of roses
The obvious question is whether there is something special about the smell of roses. Perhaps roses simply make the brain work better?
That wasn’t what the researchers found. When the scent was presented during sleep without having first been present during learning, it did not produce the same memory benefit. The smell had to have become associated with the learning experience beforehand.
Timing mattered too. Presenting the rose scent during REM sleep — the stage strongly associated with vivid dreaming — did not produce the same improvement. Nor did presenting it while participants were awake. The interesting effect occurred when the scent associated with the earlier learning experience was reintroduced during slow-wave sleep.
This suggested that the scent itself wasn’t improving memory. Rather, it appeared to be acting as a reminder to the sleeping brain.
The sleeping brain is surprisingly busy
We often talk about sleep as though the brain simply switches off for several hours and starts up again in the morning. In reality, sleep is an active biological process, and one of the important things happening during sleep is memory consolidation.
When we learn something new, the memory is initially fragile. Over time, the brain stabilises and reorganises that information, integrating it with knowledge that is already stored. Sleep appears to play an important role in this process.
The hippocampus is particularly important in the formation of new memories. During sleep, patterns of neural activity associated with recent experiences can be replayed, helping newly learned information become more firmly established in long-term memory.
This is where the roses become interesting. Because the scent had been present during learning, it had become associated with the memory. When the same smell was introduced during sleep, it appears to have encouraged the brain to reactivate the memory connected with it.
Researchers call this phenomenon targeted memory reactivation, or TMR.
Smells aren’t the only triggers
The idea isn’t quite as strange as it first appears. Most of us have experienced the extraordinary ability of a smell, song, or place to bring back a memory.
Perhaps you smell a particular perfume and immediately think of your grandmother. A song you haven’t heard for twenty years may suddenly transport you back to high school. The smell of sunscreen might bring back a childhood holiday so vividly that you can almost see the beach.
These sensory experiences have become associated with memories. Targeted memory reactivation uses a similar principle, except researchers deliberately create the association during learning and then present the sensory cue again while the person sleeps.
Since the original rose experiment, researchers have used both smells and sounds to investigate the phenomenon. Studies have examined whether these cues can strengthen different kinds of learning, including vocabulary, spatial information and motor skills.
In one particularly interesting real-life study, participants learned Japanese vocabulary with an odour present. The same odour was then used at different stages of learning, sleeping and testing. Those who experienced the odour during learning, sleep and testing showed better vocabulary learning overall than the other groups.
It seems that the connection between our senses, sleep and memory may be more powerful than we realise.
Does this mean we can learn while sleeping?
Unfortunately, this doesn’t mean that you can play a French course beside your bed and wake up speaking fluent French. Nor will putting a mathematics textbook under a child’s pillow accomplish very much.
The important point is that the information must first be learned while the person is awake. In the rose experiment, the participants had already learned the picture locations. The smell presented during sleep did not teach them anything new; instead, it appears to have helped the brain reactivate and strengthen something that had already been learned.
That distinction is important because it tells us something fundamental about memory. Learning does not end the moment we close the book. What happens afterwards matters too.
Maybe we really should “sleep on it”
People have been telling one another to “sleep on it” for generations. We usually use the expression when someone has a difficult decision to make, but modern research suggests that sleep really does give the brain time to work with information gathered during the day.
While we sleep, memories may be replayed, strengthened, reorganised and connected with existing knowledge. Scientists are still discovering exactly how these processes work, and targeted memory reactivation remains an active field of research. It certainly isn’t a magic trick for learning without effort, but it provides a remarkable demonstration of just how active memory can be while we are completely unaware of it.
Perhaps the strangest part of the rose experiment is not that a smell can influence memory. It is the reminder that when we put away our books, switch off the light and fall asleep, our learning brain has not necessarily finished its work for the day.
