While there are numerous reports on the transient effects of essential oils, the establishment of evidence concerning their efficacy on long-term mental and physical states remains insufficient. The purpose of this study is to investigate the relationship between essential oils and longitudinal changes in mental and physical stress levels through a subacute study using wearable sensors, salivary stress markers, and questionnaires. Thirteen healthy adult Japanese women participated in this study, who wore wearing portable monitors continuously for 24 h over five weekdays to measure daytime activity levels and sleep depth. The essential oil fragrance was administered by applying scented lotion at bedtime, and the relationship between the essential oil and the sleep state was examined. Subjective evaluation using a preference questionnaire indicated that the odor was generally well-received by the participants. Wearable devices were effective in visualizing experimental conditions, such as the lack of significant differences in daytime activity levels. Correlations were observed between subjective assessments from mental and physical state questionnaires and salivary amylase activity results. Additionally, sympathetic nerve activity, as measured by salivary amylase activity, significantly decreased from the first to the fourth day, indicating a reduction in mental stress level due to the fragrance presentation. As a short-term effect, the exposure of the fragrance before sleep suggested the potential to improve sleep quality by increasing the proportion of deep sleep.