Daytime sleepiness is a major symptom of obstructive sleep apnea syndrome and presents great public health concern. The underlying mechanisms are poorly understood. Poor correlation often exists between subjective and objective test measures, but new analytic methods have given hope for better results. Further study and validation of these methods, and development of alternative subjective and objective tests for sleepiness, will facilitating the clinical assessment of excessive daytime sleepiness. Compliance with the primary treatment must be maintained during provision of supplemental medication to sustain healthy outcomes.