A monitoring system monitors and/or predicts drowsiness of a driver of a vehicle or a machine operator. A set of infrared or near infrared light sources is arranged such that an amount of the light emitted from the light source strikes an eye of the driver or operator. The light that impinges on the eye of the driver or operator forms a virtual image of the signal sources on the eye, including the sclera and/or cornea. An image sensor obtains consecutive images capturing the reflected light. Each image contains glints from at least a subset or from all of the light sources. A drowsiness index can be determined based on the extracted information of the glints of the sequence of images. The drowsiness index indicates a degree of drowsiness of the driver or operator.