Does Your Brain Still Smell While You're Asleep?
Reviewed by Dr. Michael Leon, PhD, Professor Emeritus of Neurobiology and Behavior, University of California, Irvine.
Think about what you do to fall asleep. Lights off, curtains drawn, phone away, room quiet, eyes closed. Falling asleep is mostly an act of shutting your senses down one at a time.
You can't do that with smell; there is no eyelid for your nose.
Sight, sound, touch, and taste are relayed through the thalamus, a structure in the middle of the brain, on their way to the cortex, where you actually perceive them. During sleep, that relay is turned down.
Smell never goes through the thalamus. Odor signals travel from receptors in your nose to the olfactory bulb and straight into the amygdala and the entorhinal cortex and then the hippocampus — the memory and emotion centers.
So the smells in your bedroom are arriving to your brain all night. Which raises the question: what is your brain doing with them?
Sleep is not an off switch
It helps to stop thinking of sleep as something you do and start thinking of it as something your body uses.
Nighttime is when a set of jobs get done that cannot be done during the day. Memories from waking hours are replayed and filed into long-term storage. The glymphatic system — the brain's own drainage network — opens up and flushes metabolic waste out of neural tissue.
Most of this is concentrated in deep, slow-wave sleep, the deepest sleep stage of the night. And deep sleep is precisely what drops as we age: across the lifespan, slow-wave sleep declines steadily from young adulthood onward.
You can learn while asleep through smell
If sleep is when the brain files away memories, can it also form new ones? Researchers at the Weizmann Institute set out to test this using smell.
Even when we’re asleep, we inhale deeply when something smells pleasant and cut the breath short when it smells foul. Sniff depth is a window into how a sleeping brain is reacting to an odor.
Researchers tested this idea by playing a tone followed by a smell, over and over through the night. One tone was always paired with a pleasant odor, another with an unpleasant one. The next day, with participants awake, they played the tones alone, with no smells at all. Participants had no memory of the night and noticed nothing unusual, but their breathing told a different story. The tone once paired with the pleasant scent drew a deep sniff; the other drew a shallow one. Their brains had learned a new association during sleep based on smell.
Two years later, the same group asked whether this kind of sleep learning could change behavior. Sixty-six people who wanted to quit smoking spent a single night in the lab while the smell of cigarettes was paired with repellent odors like rotten fish and ammonium sulfide. Over the following week, those conditioned during stage-2 sleep smoked roughly a third fewer cigarettes than before, and the effect held through day seven. A group given the same pairing while awake showed no significant change.
In other words, the sleeping brain didn't just take in smell; it learned a new behavior from it.
Odors can deepen sleep
In addition to affecting our memory while we sleep, smell can change how deeply we sleep. A team led by Ofer Perl and Noam Sobel found that odors presented during non-REM sleep strengthened slow oscillations, the brain waves that mark deep sleep. Unlike sounds, the odors did this without waking people or disrupting their rest. Smaller studies using essential oils have pointed in the same direction.
And change dreams
If odor reaches the amygdala during sleep, you would expect it to show up in the emotional content of dreams. It does.
This notion was tested in Germany, where 40 inpatients being treated for PTSD took part in a randomized, patient-blind, placebo-controlled trial. For five nights, half received a pleasant odor while they slept and half received clean air, delivered either through a nasal tube triggered by their own breathing or through an odorized nasal clip. Each morning, the patients filled out standardized questionnaires about their sleep and their dreams, while a motion sensor tracked how long they took to fall asleep, how efficiently they slept, and how much of the night they spent awake.
The odor changed how intense their dreams were. In the group using the odorized nasal clip, the researchers recorded an improvement measured against the patients' own baseline — a moderate-to-large effect — and no negative effects at all.
What this means for your nights
Every night, you close your eyes, quiet the room, and shut your senses down one by one. Smell is the one that stays on. It keeps delivering signals straight to the brain's memory and emotion centers during the very hours when the brain is filing away memories and doing its deepest restorative work.
The research shows that sleeping brains don't just notice odors. They learn from them, sleep more deeply with them, and even dream differently because of them.
Our bedrooms are among the most scent-neutral places we spend time, which means one of the brain's most direct inputs goes unused for a third of our lives.
Memory Air was designed to change that. Built on Dr. Michael Leon's research into olfactory enrichment, it releases a rotating sequence of pleasant scents throughout the night, giving your brain steady, varied input during the hours it's already primed to use it.