Can a simple scent released while you sleep improve your sense of smell, your memory or even the quality of your sleep? New research explores the benefits of passive olfactory stimulation.One in five people experience a loss or reduction of their sense of smell. This can result from different conditions, including respiratory infections, sinusitis, COVID-19, head trauma, neurodegenerative diseases or exposure to chemicals.Although invisible, it significantly reduces quality of life.Those affected may isolate themselves for fear of smelling bad, experience anxiety, lose their appetite or change their diet. Their everyday safety can even be compromised, since detecting smoke, gas or spoiled food becomes more difficult. This loss can also hinder certain professions, such as chefs, perfumers or sommeliers, whose sense of smell is an essential tool of the trade.The main treatment for loss of smell is olfactory training. Every day, morning and evening, the protocol involves exposing oneself to different smells for around 30 to 40 seconds each.The limits of olfactory stimulationAlthough the beneficial effects of olfactory stimulation have been documented in several studies, the dropout rate is high. This is because to be effective, olfactory training requires two sessions a day for at least three months. The repetitive routine is demanding, and some people give up before they see any benefits.As an alternative, researchers have come up with a simple and accessible solution: passive olfactory stimulation. This involves continuously exposing the brain to a variety of smells, even while sleeping.Brain remodellingEffortless and drug-free, passive olfactory stimulation has attracted attention from neuroscientists who are seeing its benefits. Some studies show an improvement in memory, anatomical changes in the brain and better sleep.These effects are based on brain plasticity — the brain’s ability to reshape itself in response to experiences and learning. By smelling scents, we activate and stimulate the brain’s limbic system, a key region that regulates emotions and memory. Despite being a rapidly expanding field of research, this approach is not well known to the general public. Its beneficial effects on the brain could improve the quality of life for many people, especially as complaints of memory, sleep or smell increase with age.A memory ‘booster’The smell of a sugar tart fresh from the oven can be enough to bring back a childhood memory of one’s grandmother. Like in French novelist Marcel Proust’s In Search of Lost Time, where childhood memories are unlocked by the smell of a madeleine cake dipped in tea, this phenomenon illustrates the intimate link between smell and memory.A study published in Frontier in Neuroscience exposed a group of healthy adults aged 60 to 85 to a different scent each night — rotating through seven scents over a week — for six months. Results showed a 226 per cent improvement in verbal memory compared to a group of adults who were not exposed.At the brain level, the researchers reported an increase in the left uncinate fasciculus, a communication pathway between the regions of the brain involved in memory and learning. The fact that this structure is positively altered by passive olfactory stimulation suggests the effects may not be merely temporary.Olfactory stimulation could act as a real “booster” for the memory of older people. However, although these results are encouraging, they need to be confirmed by further studies, particularly given the small sample size used.Benefits for sleepSmells and sleep have been closely linked for thousands of years. Even in ancient Egypt, our ancestors burned myrrh incense to protect sleepers from nightmares. Today, research suggests that olfactory stimulation practised at bedtime and during the night could indeed improve sleep.Sleep is crucial for our physical and mental health. It allows our bodies to recover, regulate emotions and eliminate metabolic waste accumulated during the day, and it supports the immune system. It is also a key time for consolidating memories, transforming newly learned information into long-term memories. However, as we age, our sleep becomes more fragmented, involves more awakenings and tends to lower in quality.A meta-analysis combining several studies shows that certain scents can improve sleep quality in people with insomnia. Lavender, orange peel and rose are the most widely studied, though their respective therapeutic effects remain poorly understood.Similarly, a pilot study found that diffusing lavender at night could improve both perceived sleep and certain objective sleep indices measured by an electroencephalogram. Compared to a scent-free night, the lavender night was associated with greater well-being upon waking, fewer sleep disturbances, and an increase in deep slow-wave sleep and delta activity — a marker of deeper, restorative sleep.These results are still preliminary, but suggest that passive olfactory stimulation during the night could promote better sleep quality.Where to start?There is no need for sophisticated equipment to get started with passive olfactory stimulation. Here are a few ideas:Smell a perfume or essential oil every morning or evening on your clothes.Use a diffuser or room fragrance in your living space.Expose yourself to different scents.Rotate through at least four different scents. Use each scent for about 30 seconds in each nostril, twice a day, for three to six months.To prepare our bodies for sleep, we dim the lights, seek silence and put on loose-fitting clothes. Without even realizing it, we engage almost all our senses. So why not add a soothing scent before we close our eyes?Coline Zigrand has received funding from the Fonds de recherche du Québec en Santé (FRQS).Benoît Jobin receives funding from the Canadian Institutes of Health Research (CIHR).