To measure vigilance disorders in healthy normals or in patients (narcolepsy, sleep apnea syndrome) is difficult, time-consuming and hardly objective with present methods. Recording and analysis of spontaneous pupillary behaviour in darkness by infrared video pupillography is an objective and time-saving method to measure daytime sleepiness. However, certain external conditions must be satisfied (avoid light, noise, stress) to get reliable results. Spontaneous pupillary oscillations are recorded in darkness over 10 min and data are analyzed by fast Fourier transformation, with additional calculation of the mean pupillary diameter for each time segment (approx. 1 min). While in the alert normal, pupil remains dilated during the measurement in darkness and oscillates with an amplitude below 0.3 mm and a frequency about 1 Hz, there are characteristic changes in fatigue: (1) low-frequency components dominate the spontaneous pupillary oscillations, with an amplitude reaching several millimeters, and (2) pupil diameter decreases with time. Infrared video pupillography could play a role as a screening method and therapy control for hypersonic patients (most frequent: sleep apnea syndrome) with excessive daytime sleepiness. An objective, time-saving method like infrared video pupillography would be useful in sleep medicine and psychiatry when testing the level of vigilance, and in psychology or industrial medicine as well, providing informations about acute vigilance problems in healthy normals.