Multiple studies have shown that if you use an aroma while learning during the day and then smell the same aroma while you're sleeping, memory retention increases. That's why I use a specific scent while I read and then again in my diffuser while I'm sleeping.
How it works
When you're learning something during the day, your brain records that information in your short-term memory. When you're sleeping, your brain files the short-term memories as long-term memories. While your brain is filing the massive amount of data you take in during the day, it decides what's important enough to transfer to long-term storage. Using a scent while learning during the day followed by smelling the same scent while you sleep improves memory consolidation and performance.
Imagine that you have a 2TB hard drive of video footage from a recent trip. When you open the folder, you see a list of file names, but don't know which ones are the most important. Consider the initial recording of those experiences as your short-term memory.
While going through the footage file by file, you move your favorite clips to a separate folder. It took you hours to go through that process, but the next time you open the folder, you'll instantly be able to see your most important clips. That's because you took the time to organize your data and specified what was important enough to be more easily accessible in the future.
Your brain works in a similar way because it has to decide which information is important enough to be stored in your long-term memory. Smelling the same scent that was present during the recording of a short-term memory tells your brain that the information is important. That leads to the specific information being recalled easier.
A recent study found that students improved their learning by 30% when they smelled a rose aroma while learning and then again while they slept. Of course, this study was done in a controlled manor, but their results indicate that odor cueing throughout the night provides a similar effect as highly controlled lab studies. In addition, previous studies concluded the same positive results, but specifically noted that the benefits of odor cues occurs during slow-wave sleep.
One of my favorite facts mentioned in the study explains that we transfer 1 to 10 Mbit of information per second from our eyes to the primary visual cortex. That's roughly 7.5MB to 75MB of data per minute. To give you an idea of how much data that is, an average MP3 is 4MB and a 4K movie is 4000MB.
If your brain is transferring data on the higher end at 75MB per minute, that's about one 4K movie per hour or 16 each day, assuming you get 8 hours of sleep. If you're transferring data on the low end, it would be closer to 2 movies. And that's just the visual data being transferred. Your brain is receiving data from all of your other senses at the same time!
However, only a fraction of this information enters consciousness. And only a fraction of the conscious (but also partly unconscious) information in turn enters long-term memory.
One way to help the brain determine what information is stored in long-term memory is by using odor cues. That means using a scent while you learn something and then again while you sleep.
Using this information to your advantage
To take advantage of these benefits, I use a specific aroma when I'm reading and then diffuse that same scent while I sleep. I keep my diffuser on interval mode so it's not constantly running throughout the night, but enough to have the scent present for most of my sleep.
Give it a shot for yourself. Try reading with a unique scent and set your diffuser to interval mode throughout the night. After a couple days of doing that, do you feel like you retained more information?
If you want to learn more about odor cues this study is quite extensive.