Daytime somnolence is both a symptom in many patients and a prevalent complaint in the general population, but its objective assessment remains elusive. The current available tests are technically complex and thus inadequate for routine clinical use or epidemiological studies. A simplified behavioral maintenance of wakefulness test (OSLER test) has been recently described that could allow for widespread availability of objective measurements of this symptom. We verified the occurrence of (micro)sleep (episodes of sleep of > or = 3 s duration) during the performance of the OSLER test in 10 normal subjects after a non-sleep-deprived night and a sleep-deprived night in randomized order. Sleep was assessed electrophysiologically according to standard methods. The OSLER test (mean of four measurements) was significantly shorter after the sleep-deprived night (25 min versus 38 min). Single missed stimuli were frequent with or without (micro)sleep, but (micro)sleep was almost always present when four or more consecutive stimuli were missed. The sensitivity and specificity of the test in detecting sleep (of > or = 3 s duration) are 85% and 94%, respectively. The total number of missed stimuli per minute duration of the test could add valuable information to the simpler mean test result. Sensitivity and specificity are not altered when only three measurements are performed. We conclude that the OSLER test appears as a simple, easy, and reliable method to objectively assess daytime somnolence.