The method and system for detecting operator alertness of an operator of a vehicle or machine comprises an image collection system. The image collection system collects reference motion data associated with an operator representation when the operator is in an alert state. An image processor determines observed motion data of one or more points of a three dimensional representation of the operator during a time interval. An analyser sends an alert signal to alert the operator if a detected angular shift of one or more points of the representation exceed a motion threshold.