The present invention generally relates to a method for improving the reliability of a portion of physiological data from an image sensor monitoring an operator positioned in an operator compartment of a vehicle. An embodiment of the method includes receiving, from the image sensor, physiological data including information relating to at least one of eye, face, head, arms and body motion of the operator, identifying an indication of at least an eyelid closure, eye movement or head movement of the operator based on the physiological data; comparing at least one of the physiological data and a lighting condition within the operator compartment with a set of rules for a current operator status; and classifying the type of eyelid closure, eye movement and/or head movement by correlating the identified eyelid closure, eye movement and/or head movement and a result of the comparison.