A method and a system for quantifying anaesthesia and/or a state of vigilance (e.g. monitoring sedation or sleep) from a plurality of parameters acquired from a subject allows to determine an indicator that reliably quantifies the hypnotic component of anaesthesia and/or the state of vigilance and/or the analgesic component of anaesthesia, even if the number of parameters varies while monitoring the subject. Preferably, a first sub-indicator based on a first subset of parameters adapted to characterise a boundary region between consciousness and unconsciousness is determined, and a second sub-indicator based on a second subset of parameters adapted to characterise a level of hypnosis of said subject is determined, and said first sub-indicator and said second sub-indicator are then combined to compute a global indicator value.
