A total of 47 patients with sleep disorder (36 male and 11 female) with a mean age of 47.5 +/- 15 years were evaluated for daytime symptoms with a Multiple Sleep Latency Test (MSLT) and a Maintenance of Wakefulness Test (MWT) given on the same day--once at the time of their diagnostic evaluation and again after one to six months of treatment. The MSLT and MWT data are consistent with the notion that sleep tendency, as measured by the MSLT and ability to remain awake, as measured by the MWT, represent different physiologic processes. Data show a marked treatment-related improvement in ability to stay awake as measured by the MWT and no treatment-related improvement in sleepiness as measured by the MSLT. We conclude that there is a heterogeneous subpopulation of patients with sleep disorders whose symptoms of daytime sleepiness will show no treatment-related improvement in daytime symptoms if they are evaluated only by the MSLT. We suggest that, since ability to stay awake (and not ability to fall asleep) is a requisite for all job-related duties, an objective, physiologically based test such as the MWT should be used to assess the impact of sleep disorders in cases where there is a clinical concern about fitness to drive or work.