A head worn device for monitoring alertness and attention which includes a) sensors to monitor eyelid and eye movement, b) a motion sensor and c) data storage means for storing data from said sensors. The motion sensor is an accelerometer to provide data that allows the head position to be analysed and to determine the direction of gaze. The device collects data from an accelerometer worn by the driver to detect if the vehicle is in motion and whether the head is tilted vertical plane. If the vehicle is not in motion data is not processed. When the direction of gaze is downward sensor signals from the eye movement sensors is ignored. However if the duration of downward gaze is greater than a predetermined minimum period and the vehicle is in motion, an alarm is triggered because the driver is inattentive. The device may be used to assess whether workers are sufficiently alert before they commence work. The method of measuring a subjects fitness for a particular task uses the alertness monitor or a video camera to monitor eye and eyelid movement with or without head movement data while the subject is tested with a series of tests that require the subject to follow predetermined images or lights. If the subject is unable to satisfactorily track the images the subject is not fit for work.Linvention concerne un dispositif porté sur la tête, pour surveiller la vigilance et lattention, qui comprend a) des capteurs pour surveiller le mouvement des paupières et des yeux, b) un capteur de mouvement et c) un moyen de stockage de données pour stocker les données provenant desdits capteurs. Le capteur de mouvement est un accéléromètre pour fournir des données qui permettent d’analyser la position de la tête et pour déterminer la direction du regard. Le dispositif rassemble des données provenant de laccéléromètre porté par le conducteur pour détecter si le véhicule est en déplacement et si la tête est inclinée ou non par rapport à un plan vertical. Si le véhicule ne se déplace p
