An alertness monitoring system includes: means to measure velocity and amplitude of eyelid and eye movement; and storage means to continuously record the measurements. A data processor calculates the blink amplitude to velocity ratio and/or the saccade amplitude to velocity ratio and compares the measurements against a predetermined scale. The scale is derived from averaged measurements of the eye and eyelid movement parameters for a sample population measured against predetermined incremental blood alcohol levels. The output of the system can include a display for showing the alertness measurement based on the scale, or an alarm means triggered by the scale reading reaching a predetermined limit based an the blood alcohol level scale.