The OSLER test represents a simple alternative to the maintenance of wakefulness test. Standard analysis of OSLER test results yields a mean sleep latency (MSL). The aim of this study was to use both MSL and errors (nonresponses to stimulations) to characterize daytime sleepiness in apneic patients. OSLER test results at 9:00 A.M., 11:00 A.M. and 1:30 P.M. were compared in 27 obstructive sleep apnea syndrome patients (50.4 +/- 10.4 years; apnea-hypopnea index: 43.05 +/- 25.08) and 20 control subjects (C). Not only did obstructive sleep apnea syndrome patients demonstrate earlier sleep onset than control subjects (MSL: 1,788 seconds +/- 511 versus 2,335 seconds +/- 139, p < 0.001), but they also spent a greater percentage of time making errors than control subjects (5.4% +/- 4.7 of total test time versus 0.4% +/- 0.4, p < 0.001) with specific error profiles (high prevalence of three to six consecutive errors). When error profile analysis was added to standard sleep latency assessment, up to 40% of patients with normal sleep latency were exhibiting abnormal fluctuation in vigilance. A single 9:00 A.M. OSLER session appeared as sensitive as three consecutive sessions in its use as a means of identifying patients with significant daytime sleepiness. On the other hand, the 1:30 P.M. OSLER test session was least specific to distinguish apneic subjects from normal subjects, suggesting that the OSLER test can identify the afternoon peak in physiologic somnolence.