An odor that is hard to notice in the morning may seem easier to detect in the evening. Can the same person’s sensitivity to odors vary over the course of a day?
In 2023, Yiling Mai and colleagues tested 56 adults four times in one day, from morning to evening. Their aim was to examine whether odor sensitivity changes with time of day and whether nasal airflow and individual differences in daily rhythms, such as morning or evening chronotype, are related to those changes. The study explored whether measurement time could influence results relevant to olfactory assessment and treatment.
Testing the ability to detect faint odors
The main sample comprised 56 adults aged 18–68: 32 women and 24 men, with an average age of about 31. All reported having a normal sense of smell, but 16 met the criteria for reduced olfactory function on the initial comprehensive test. This mismatch between self-perception and test results is important when interpreting the findings.
Testing took place during four time windows: 8–10 a.m., noon–2 p.m., 4–6 p.m., and 8–10 p.m. The initial assessment tested the faintest detectable odor, discrimination between odors, and odor identification. The subsequent three sessions repeatedly measured sensitivity to faint odors.
This sensitivity is expressed as the olfactory detection threshold. In general, the lower the concentration someone can detect, the more sensitive they are. However, the score used in this test increases with sensitivity. It is important not to confuse an increase in the score with an increase in the concentration needed to detect an odor.
The researchers also measured airflow during a maximal nasal inhalation, blood pressure, pulse, and attention. Participants rated their sense of smell, nasal patency, sleepiness, and other states. Morning and evening chronotypes were assessed with a questionnaire asking about preferred times for activity and related habits.
Mean scores were higher in the evening than in the morning
Mean scores for sensitivity to faint odors were 6.68 in the morning, 7.33 at noon, 7.10 in the afternoon, and 7.63 in the evening. There was a statistically significant difference across the four time windows. In individual comparisons, the evening score was significantly higher than the morning score by 0.95 points. Other comparisons involving noon and afternoon did not reach significance.
This means that participants were more sensitive in the evening on average. Not everyone showed the same change, and the score difference cannot be converted into a claim that people smell a certain number of times better at night. The test repeated four times measured detection threshold, so the results do not show that fragrance preferences or the ability to identify ingredients in a complex perfume also improve in the evening.
Participants’ self-ratings of their sense of smell did not differ significantly by time of day. Small changes detected by testing did not necessarily translate into a consciously noticed difference.
Nasal airflow also increased, but the changes were not equivalent
Mean airflow during a maximal nasal inhalation was 72.55 liters per minute in the morning and 90.41 liters per minute in the evening. This too was higher in the evening, indicating changes in nasal patency with time of day.
Did easier airflow through the nose therefore make participants more sensitive to odors? When the team compared each person’s change from morning to evening, there was no significant correlation indicating that larger increases in airflow accompanied larger increases in threshold score. Airflow and threshold score were also not significantly correlated at any individual time point.
The authors consider the possibility that nasal patency and olfactory sensitivity each have their own patterns of change. However, nasal airflow and odor delivery are not necessarily in a simple proportional relationship, and the measurement method has limitations. Nonsignificant correlations alone do not establish that nasal patency is unrelated to smell.
Morning and evening chronotypes offer preliminary clues
In a statistical model accounting for other conditions, morning testing, evening chronotype, and BMI calculated from self-reported height and weight were among the factors associated with threshold scores. Evening types tended to score lower than morning types. Higher BMI was also associated with higher scores, but this is not evidence that gaining weight improves smell.
By contrast, an analysis of whether the pattern of daily change differed by chronotype across all four time windows found no significant difference. An additional analysis combining morning with noon and afternoon with evening then found a difference in the pattern of change between chronotypes.
Scores tended to be higher earlier in the day for morning types and later for evening types, but the early–late difference within each chronotype was not significant. With only nine evening types and 14 morning types, the findings are insufficient to conclude that morning is optimal for morning types and evening for evening types. The authors call for further research into whether individual differences in daily rhythms also influence olfaction.
The effect of the body clock was not isolated
Testing was repeated in chronological order from morning onward on the same day. Changes associated with time of day cannot be fully separated from growing familiarity with the test. An exploratory assessment in four other participants measured the four time windows on separate days, but did not show the same pattern as in the main group of 56. Four participants cannot rule out an order effect.
The study also did not cover the full 24-hour cycle, including late night, and did not measure hunger or satiety, hormonal state, menstrual cycle, or related factors. Unlike experiments that strictly control sleep and light to investigate the body clock, this study observed changes over an ordinary day.
The authors discuss potential clinical questions, such as whether considering test time could reduce missed cases of olfactory impairment or whether the timing of olfactory training affects its benefits. The study did not compare diagnostic accuracy or treatment success by time of day, so an established basis for changing assessment or treatment schedules has not yet been provided.
The acknowledgments list support from the Volkswagen Foundation and a scholarship from the China Scholarship Council. The authors declared no financial conflicts of interest.
Summary
In these 56 participants, mean scores on a test of detecting faint odors were higher in the evening than in the morning. Nasal inspiratory airflow was also higher in the evening, but the two changes did not correspond in a simple way. Individual differences such as chronotype may also be relevant.
Treating a single smell-test result as someone’s unchanging ability may overlook something important. Separating time of day and daily rhythms from nasal patency and test familiarity is the next step toward understanding olfactory fluctuations and, eventually, improving assessment and treatment.
References
Mai, Y., Rosbach, M. C., & Hummel, T. (2023). Variations of olfactory function with circadian timing and chronotype. Rhinology, 61(5), 456–469. DOI: 10.4193/Rhin23.150. Full original article PDF.
