Clinical approach to excessive daytime sleepiness.

Subjective sleepiness is defined as "an elemental feeling state" by which the subject, and only him/her knows whether he/she is alert or sleepy. Subjective sleepiness has little been used for the clinical evaluation of sleepiness. Objective sleepiness is defined as a consequence of the homeostatic need for sleep. In the absence of a measurable marker of the fulfillment of the function of sleep, the evaluation of objective sleepiness has to rely on a behavioral approach based on the postulate that the more you need sleep (i.e., the sleepier you are), the more likely you are to fall asleep. However, as for many other fundamental human behaviors (drinking, eating, sexual behavior), it is likely that the sleeping behavior serves other purposes than its primary objective, which distorts the relationship between the observed sleeping behavior and the actual need for sleep. The likelihood to fall asleep can be measured by the speed with which a sleep episode occurs under standardized laboratory conditions (multiple sleep latency test, maintenance of wakefulness test, OSLER test), or by the frequency of unwanted sleep episodes under the more naturalistic conditions of daily life (Epworth Sleepiness Scale). These various approaches have advantages and drawbacks, but none of them can be considered the "gold standard".