Obstructive sleep apnea (OSA) is a common condition most often characterized by daytime sleepiness. cPAP therapy is very effective in reversing symptoms. However, some patients still complain of excessive daytime sleepiness despite treatment, It is essential to document sufficient use of CPAP and to rule out the presence of residual respiratory events. Search for other underlying causes of sleepiness are then to be investigated by a detailed history together with a sleep night recording. The purpose of this article is to guide the clinician in providing comprehensive medical care for those patients.