A 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 position data and reference motion data associated with an operator, or a portion thereof, when the operator is in an alert state. An image, processor determines observed position data and observed motion data of one or more points of a three dimensional representation of the operator during a time interval. An analyzer sends an alert signal to alert the operator if a detected angular shift of one or more reference points of the representation exceeds at feast one of a displacement threshold and a motion threshold.