Abstract: OBJECTIVE To investigate the influence of circadian rhythm on anesthesia depth of propofol. METHODS Patients undergoing general anesthesia within 8：00~12：00 or 22：00~2：00 were chosen as day group and night group (n=30 each group). All the patients aged 20~45 years without sleep disorder. Anesthesia was induced with propofol delivered by target controlled infusion(TCI). The initial effect-site concentration of propofol was set at 1.0 μg·mL^-1, and increased step-by-step with 0.5 μg·mL^-1 after having balanced for 2 min until reached 3.0 μg·mL^-1. And then patients were injected with midazolum 0.05 mg·kg^-1, fentanyl 5 μg·kg^-1 and cisatracurium 0.15 mg·kg^-1 by intravenous before tracheal intubation. The bispectral index(BIS), MAP and HR values were recorded before propofol infused(T₀), at each effect-site concentration of propofol (T₁: 1.0 μg·mL^-1; T₂: 1.5 μg·mL^-1; T₃: 2.0 μg·mL^-1; T₄: 2.5 μg·mL^-1; T₅: 3.0 μg·mL^-1) and before tracheal intubation(T₆). RESULTS The BIS values were significantly lower at T₃, T₄, T₅ and T₆ in night group compared with day group(P<0.05). The MAP and HR were gradually decreased when induction, but without significant difference between the two groups(P>0.05). CONCLUSION Circadian rhythm may have remarkable effect on the depth of propofol anesthesia, and it is more potent when administered at night.