Effects of nighttime odor exposure and delivery methods on subjective sleep quality in healthy adults

The role of smell in improving sleep quality is inconclusive. This study aimed to investigate whether the type of odor and the method of application affect sleep quality. Based on individual preference 112 healthy participants (mean age ± SD = 31 ± 12 years, 43 men) selected an odor (lavender, orange, specifically designed "perfume") and an application method (odorized nasal clips, odors on pillow, odors through diffuser). Their performance was compared to a control group (n = 19; mean age ± SD = 29 ± 6 years, 9 men). Except for the control group, all groups slept with exposure of odor for 2 weeks (phase 1), followed by 2 more weeks of normal sleep without odors (phase 2). The Pittsburgh Sleep Quality Index (PSQI) and the wearable sleep monitoring device "Fitbit Charge 2" were used to measure sleep quality both subjectively and objectively. After adjusting for baseline differences, participants exposed to "perfume" reported better subjective sleep quality during the odor exposure phase compared to the control group. Among the application evaluated, the use of a diffuser was associated with better subjective sleep quality and reduced daytime fatigue, as reflected in the PSQI components. No significant group differences were observed in objective sleep parameters measured by Fitbit. Exposure to odors during the night influence sleep quality. Using a diffuser to deliver the odor appears to be associated with better subjective sleep quality and reduced daytime fatigue. No sustained improvement in sleep quality over time was observed with odor exposure.