Literature Review

Inhaling pleasant scents during sleep tied to a dramatic boost in cognition



Inhaling a pleasant aroma during sleep has been linked to a “dramatic” improvement in memory, early research suggests.

In a small, randomized controlled trial researchers found that when cognitively normal individuals were exposed to the scent of an essential oil for 2 hours every night over 6 months, they experienced a 226% improvement in memory compared with a control group who received only a trace amount of the diffused scent.

In addition, functional magnetic resonance imaging (fMRI) showed that those in the enriched group had improved functioning of the left uncinate fasciculus, an area of the brain linked to memory and cognition, which typically declines with age.

“To my knowledge, that level of [memory] improvement is far greater than anything that has been reported for healthy older adults and we also found a critical memory pathway in their brains improved to a similar extent relative to unenriched older adults,” senior investigator Michael Leon, PhD, professor emeritus, University of California, Irvine, said in an interview.

The study was published online in Frontiers of Neuroscience.

The brain’s “superhighway”

Olfactory enrichment “involves the daily exposure of individuals to multiple odorants” and has been shown in mouse models to improve memory and neurogenesis, the investigators noted.

A previous study showed that exposure to individual essential oils for 30 minutes a day over 3 months induced neurogenesis in the olfactory bulb and the hippocampus.

“The olfactory system is the only sense that has a direct ‘superhighway’ input to the memory centers areas of the brain; all the other senses have to reach those brain areas through what you might call the ‘side streets’ of the brain, and so consequently, they have much less impact on maintaining the health of those memory centers.”

When olfaction is compromised, “the memory centers of the brain start to deteriorate and, conversely, when people are given olfactory enrichment, their memory areas become larger and more functional,” he added.

Olfactory dysfunction is the first symptom of Alzheimer’s disease (AD) and is also found in virtually all neurological and psychiatric disorders.

“I’ve counted 68 of them – including anorexia, anxiety, [attention-deficit/hyperactivity disorder], depression, epilepsy, and stroke. In fact, by mid-life, your all-cause mortality can be predicted by your ability to smell things,” Dr. Leon said.

Dr. Leon and colleagues previously developed an effective treatment for autism using environmental enrichment that focused on odor stimulation, along with stimulating other senses. “We then considered the possibility that olfactory enrichment alone might improve brain function.”

Rose, orange, eucalyptus …

For the study, the researchers randomly assigned 43 older adults, aged 60-85 years, to receive either nightly exposure to essential oil scents delivered via a diffuser (n = 20; mean [SD] age, 70.1 [6.6] years) or to a sham control with only trace amounts of odorants (n = 23; mean age, 69.2 [7.1] years) for a period of 6 months.

The intervention group was exposed to a single odorant, delivered through a diffuser, for 2 hours nightly, rotating through seven pleasant aromas each week. They included rose, orange, eucalyptus, lemon, peppermint, rosemary, and lavender scents.

All participants completed a battery of tests at baseline, including the Mini-Mental State Examination (MMSE), which confirmed normal cognitive functioning. At baseline and after a 6-month follow-up, participants completed the Rey Auditory Verbal Learning Test (RAVLT) as well as three subsets of the Wechsler Adult Intelligence Scale–Third Edition (WAIS-III).

Olfactory system function was assessed using “Sniffin Sticks,” allowing the researchers to determine if olfactory enrichment enhanced olfactory performance.

Participants underwent fMRI at baseline and again at 6 months.

Brain imaging results showed a “clear, statistically significant 226% difference between enriched and control older adults in performance on the RAVLT, which evaluates learning and memory (timepoint × group interaction; F = 6.63; P = .02; Cohen’s d = 1.08; a “large effect size”).

They also found a significant change in the mean diffusivity of the left uncinate fasciculus in the enriched group compared with the controls (timepoint × group interaction; F = 4.39; P = .043; h 2 p = .101; a “medium-size effect”).

The uncinate fasciculus is a “major pathway” connecting the basolateral amygdala and the entorhinal cortex to the prefrontal cortex. This pathway deteriorates in aging and in AD and “has been suggested to play a role in mediating episodic memory, language, socio-emotional processing, and selecting among competing memories during retrieval.”

No significant differences were found between the groups in olfactory ability.

Limitations of the study include its small sample size. The investigators hope the findings will “stimulate larger scale clinical trials systematically testing the therapeutic efficacy of olfactory enrichment in treating memory loss in older adults.”


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