Imagine studying for a test while the faint smell of roses fills the room. That night, after you have fallen asleep, the same scent is introduced into your bedroom. Could the smell somehow help you remember what you learned earlier?
It sounds unlikely, but in 2007, researchers led by Björn Rasch tested something remarkably similar. Their experiment would reveal a curious connection between smell, sleep, and memory.
The rose experiment
Participants learned the locations of pairs of pictures on a computer screen while being exposed to the scent of roses. Later, when they entered slow-wave sleep, the researchers exposed them to the same scent again.
When their memories were tested afterward, 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 memory.
The roses themselves, however, weren’t responsible for the improvement. When the scent was introduced during sleep without having been present during the original learning, the same benefit did not occur. Timing mattered too: presenting the scent during REM sleep or while participants were awake did not produce the same result. The scent had to be associated with the learning experience and then reintroduced during slow-wave sleep.
Memories don’t sleep when we do
Sleep was once thought of largely as a period of rest, but we now know that the sleeping brain remains remarkably active. One of its important tasks is memory consolidation, the process through which newly acquired information is stabilized, reorganized, and integrated with existing knowledge.
During sleep, patterns of brain activity associated with recent experiences can be replayed. In the rose experiment, the scent appears to have acted as a cue, encouraging the sleeping brain to reactivate the memory associated with it. Scientists call this targeted memory reactivation, or TMR.
When a smell unlocks the past
The principle is not entirely unfamiliar. A particular perfume may suddenly remind you of someone you haven’t thought about for years. The smell of sunscreen may transport you to a childhood vacation, while the aroma of a particular meal can bring back a kitchen, a conversation, and people who once sat around the table.
Smell has unusually close connections with brain regions involved in memory and emotion, which helps explain why odors can trigger such vivid memories. Researchers studying targeted memory reactivation are, in effect, deliberately creating a sensory cue during learning and then using that cue later to encourage the brain to revisit the associated memory.
More than roses
Researchers have since experimented with sounds as well as smells and have investigated targeted memory reactivation in vocabulary learning, spatial memory, motor learning, and other areas. The findings vary, but they suggest that under certain conditions, a sensory cue presented during sleep can influence the consolidation of something learned earlier.
This doesn’t mean, however, that any smell or sound can magically improve memory. The cue must first become associated with the information while the person is awake. It is the connection between the cue and the existing memory that appears to matter.
Can we learn while we sleep?
Unfortunately, this doesn’t mean you can play a French course beside your bed and wake up speaking fluent French. The participants in the rose experiment first had to learn the information while they were awake. The scent did not teach them anything new; it appears to have helped strengthen a memory that already existed.
That distinction is important. Sleep does not replace learning, but it may form part of the learning process. What happens after we close the book can influence how well the information is remembered.
Perhaps that is what makes the rose experiment so fascinating. While we sleep, seemingly disconnected from the world around us, the brain continues working with the experiences of the day. And, under the right circumstances, something as simple as the scent of a rose may call one of those memories back into action.
“The roses don’t teach the sleeping brain. They simply remind it what not to forget.” — Stanley Armani
The Scent of Memory: How Roses Can Help the Sleeping Brain Remember was authored by Stanley Armani. Stanley writes about the brain, learning, and the hidden patterns that shape how we think. His work explores the strange, the hopeful, and the extraordinary sides of human potential.
