Definition reprinted with permission from Culebras, Antonio, Clinical Handbook of Sleep Disorders, Butterworth-Heinemann
A subjective report of difficulty in maintaining the alert awake state, usually accompanied by a rapid entrance into sleep when the person is sedentary. May be due to an excessively deep or prolonged major sleep episode. Can be quantitatively measured by use of subjectively deemed rating scales of sleepiness, or physiologically measured by electrophysiologic tests such as the multiple sleep latency test (see MSLT). Most commonly occurs during the daytime; however, excessive sleepiness may be present at night in a person who has the major sleep episode during the daytime, such as a shift worker.