Daytime sleepiness and cognitive functioning in sleep apnea.

Our earliest findings relating sleepiness to cognitive function revealed that, among patients with the symptom of excessive somnolence, sleep apneics were the most impaired on cognitive tasks. Although the multiple sleep latency test (MSLT) has been the standard diagnostic test for assessing daytime sleepiness, the maintenance of wakefulness test (MWT) has clinical advantages over the MSLT when the assessment of daytime alertness is the primary goal. A number of studies on patients with sleep apnea and narcolepsy indicate that the MWT is more sensitive to treatment-related improvements in sleepiness. However, sleep tendency, as measured by the MSLT, and ability to remain awake, as measured by the MWT, probably represent the same physiological process viewed from different perspectives. Some patients, particularly those who have received suboptimal treatment, will show no treatment-related improvement in daytime sleepiness if they are evaluated only by the MSLT. We believe that the MWT and MSLT measure different aspects of the central problem of abnormal sleep tendency. The MWT may be a useful adjuvant daytime test in clinical situations where it is necessary to quantify degree of impairment or effectiveness of treatment.