CONTEXT: Essential oils (EOs) have been reported to possess pharmacological properties, of which those related to the central nervous system have been especially attributed to mono- and sesquiterpenes. Baccharis uncinella DC. (Asteraceae) is used by the Laklaño Indians (Santa Catarina, Brazil) for sedative purposes. Interestingly, the species does not seem to be used medicinally elsewhere in Brazil. OBJECTIVE: This study was designed to compare the composition and sedative properties of B. uncinella EOs obtained closer (BU-SC) and farther (BU-PR) to the Laklaño Indian Reserve. MATERIALS AND METHODS: BU-SC and BU-PR obtained by hydrodistillation were analyzed by CG-MS. Mice treated with BU-SC and BU-PR (50 and 100 mg/kg) were evaluated regarding pentobarbital-induced sleeping time, body temperature, and locomotion. RESULTS: BU-SC presents a higher monoterpene/sesquitherpene ratio (0.31); α-pinene (6.42%), limonene (7.21%), caryophyllene (26.13%), spathulenol (13.39%) and caryophyllene oxide (13.26%) were identified as major components. BU-PR presents a low monoterpene/sesquitepene ratio (0.004); spathulenol (32.93%), caryophyllene oxide (27.78%), viridiflorol (5.29%) and α-cadinol (2.42%) were identified as the main components. Both samples significantly (p < 0.05, ANOVA) decreased locomotion and body temperature, as well as increased sleeping time. The hypnotic activity was sensitive to the differences in monoterpene composition. CONCLUSIONS: In comparison with a sample collected in Paraná State, B. uncinella EO collected closer to the Laklaño Indians possess a composition that better justifies the claimed sedative properties. The study confirms the value of traditional information to guide bioactivity assessment in medicinal plants, and gives notice to the ecological factors that can interfere with the conclusions of such assessments.