Reviewed by Dr. Michael Leon, PhD, Professor Emeritus of Neurobiology and Behavior, University of California, Irvine.
What Is Olfactory Enrichment?
Many of us think of the sense of smell as a sensory pleasure. Coffee in the morning, aromatherapy in a spa, or the perfume of someone we love. Very few of us think of it as something the brain actually needs.
Olfactory enrichment is the practice of exposing yourself to many different scents regularly, specifically to stimulate the brain. Not one scent, and not a scented room — a changing set of them, encountered separately, over months.
So what does that actually do to a brain? And how strong is the evidence?
Where the Research Came From
Dr. Leon has studied the olfactory system since 1970. His first discovery was that newborn rats orient to their mother's specific odor — before they can see or move well, their smell was their primary sense. Newborn humans also learn to become attracted to the scent of their mother.
That led his lab to environmental enrichment: the finding that brains exposed to a richer, more varied sensory world develop and function better than brains that aren't.
He and his colleague Dr. Cynthia Woo applied the idea to children with autism through a program called Sensory Enrichment Therapy, which uses short, daily sensory-motor exercises to stimulate brain plasticity. In their 2013 trial, parents did 4 to 7 exercises with their child twice a day, using everyday household items to engage multiple senses—touch, temperature, sight, and movement—and they switched to a new set of exercises every two weeks. After six months, 42% of the children in the program showed significant improvement in their autism symptoms, compared with 7% of the children in standard care.
The sense of smell was at the center of the program. Every day, parents also placed a drop of one of several essential oils on a cotton ball and let their child sniff it while gently rubbing their back. There were seven oils, including lavender, lemon, sweet orange, and vanilla, and the scent changed each day. At night, children slept with a scented cotton ball in their pillowcase.
Over time, it became clear that smell was carrying most of the effect. As Dr. Leon put it, "data from lab mice indicated that the entire enrichment program could be replaced by olfactory enrichment alone." That realization pointed him to his next research about the involvement of olfactory enrichment in memory at UC Irvine.
Olfactory Enrichment and Memory
Olfactory decline shows up at the same time as cognitive decline; both are associated with the loss of gray matter in the hippocampus. Moreover, olfactory loss predicts which healthy older adults will develop mild cognitive impairment, the first step on the road to dementia. Olfactory loss also predicts which people with mild cognitive impairment go on to develop Alzheimer's. And among those with Alzheimer’s, it predicts who will descend rapidly into their dementia. So, Dr. Leon wanted to test the hypothesis: can olfactory enrichment decrease or reverse cognitive decline?
The trial was published in Frontiers in Neuroscience in 2023. Here is how it was run:
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43 adults, ages 60 to 85, all with normal cognition at the start
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For six months, a diffuser ran every night for two hours after they went to bed
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Seven essential oils, including rose, orange, eucalyptus, lemon, peppermint, rosemary, and lavender, one per night, rotating through the week
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The control group got the same diffuser on the same schedule, filled with distilled water and a trace of odorant too faint to detect
The results:
Memory. Both groups took the Rey Auditory Verbal Learning Test, the gold-standard word-list memory task. The enriched group scored 226% higher on that test than the control group.
Brain structure. MRI showed a measurable change in the left uncinate fasciculus in the enriched group compared to controls. That is the pathway connecting the memory structures of the temporal lobe to the decision-making regions at the front of the brain, and it normally deteriorates with age.
Sleep. The enriched group logged about 22 more minutes of sleep per night by the end of the study, and controls logged three minutes less.
What Else Does Olfactory Enrichment Improve?
Beyond the UC Irvine trial, the effects of olfactory enrichment have been measured to show other benefits:
Dementia Patients. A team at a Korean University gave patients with moderate dementia 40 different odorants, twice a day, for 15 days. The enriched group improved in memory, attention, verbal fluency, language, odor identification, and symptoms of depression.
Deeper sleep. Odors delivered during non-REM sleep have been shown to increase slow-wave activity, the deepest stage of sleep, without producing the arousals that nighttime sound causes.
Dreams. At TU Dresden, 40 inpatients being treated for PTSD received either a pleasant odor or clean air across five nights, in a randomized, patient-blind, placebo-controlled trial. Nocturnal odorization changed their dreams to be more pleasant, with a moderate-to-large improvement in one delivery group measured against their own baseline, and no negative effects.
Mood. In a 2022 Frontiers in Neuroscience review, Leon and Cynthia Woo identified four human studies in which added olfactory stimulation was associated with improved depressive symptoms. This is consistent with the anatomy, since one of the olfactory system's first stops is the amygdala, the emotional center of the brain.
Motor coordination: In a Japanese randomized controlled trial of 145 nursing home residents, continuous lavender olfactory stimulation was associated with a significantly lower rate of falls over twelve months —roughly 43% fewer falls.
Olfactory Enrichment Delivered At Your Bedside
The protocols that produce these results are not things people will realistically do on their own. Seven scents a night for six months, or even harder, forty odors twice a day, is a part-time job.
"It's unrealistic for patients to open forty bottles of perfume and sniff each one twice every day." — Dr. Michael Leon
This is why he created Memory Air. It is a bedside device that delivers 40 distinct scents across the night, automatically, on a belt that advances so the scents never merge into a single smell. It operates while you sleep and asks nothing of you beyond going to bed.
The Bottom Line
Olfactory enrichment is a cognitive health intervention backed by a controlled human trial, brain imaging, and a plausible mechanism. The evidence so far is strongest for memory in older adults, with other supporting findings in sleep depth, dream intensity, and mood.