For verifying the phase lag entropy as the tool for measurement of level of consciousness, the investigators compared phase lag entropy to bispectral index. Using Target-Controlled Infusion pump, propofol was infused for sedation during the spinal anesthesia. The investigators measured Observer's Assessment of Alertness/Sedation Scale as the propofol concentration rises. At the same time, The investigators observed phase lag entropy score and bispectral index score.