Somnolence is known to be a major cause of various types of accidents, and ocular parameters are recognized to be reliable physiological indicators of somnolence. We have thus developed an experimental somnolence quantification system that uses images of the eye and that produces a level of somnolence on a continuous numerical scale. The aim of this paper is to show that the level of somnolence produced by our system is well related to the level of performance of subjects accomplishing three reaction-time tests in different sleep conditions. Twenty seven subjects participated in the study and images of their right eye were continuously recorded during the tests. Levels of somnolence, reaction times (RTs), and percentages of lapses were computed for each minute of test. Results show that the values of these three parameters increase significantly with sleep deprivation. We determined the best threshold on our scale of somnolence to predict lapses, and we also shown that correlations exist with some of the ocular parameters. Our somnolence quantification system has thus significant potential to predict performance decrements of subjects accomplishing a task.