Sleepiness, tiredness and fatigue are complaints which must be thoroughly analyzed to eliminate blur and ambiguity. Physiological sleepiness (“sleep pressure”) increases while being awake and additionally underlies the circadian rhythm with a lower threshold to fall asleep during night time. Excessive daytime sleepiness (EDS) is considered normal only after sleep deprivation. Clinically, EDS manifests by frequents daytime napping and/or reduced alertness with automatic behavior or - in its extreme form - in recurrent attacks of sudden, uncontrollable compulsion to sleep also in inappropriate situations ( “sleep attacks”). EDS is “objectively” addressed by measuring the mean sleep latency to four to five nap opportunities throughout the day using the multiple sleep latency test (MSLT) or the maintenance of wakefulness test (MWT). EDS denotes both, a ready entrance into sleep as well as difficulty in staying awake during daytime or accordingly in inappropriate situations. These two partially independent aspects of EDS are separately assessed by the “passive” MSLT and the “active” MWT respectively. For that reason the MSLT and MWT only weakly correlate with each other when tested over a broad range of patients with EDS. It is important to keep in mind, that these tests are importantly influenced by a great variety of factors such as mood, anxiety, and motivation. “Vigilance” comprises wakefulness, alertness and attention and therefore is more than just the reciprocal to sleepiness. Cognitive performance tasks such as Steer Clear Reaction Time Test (SCRTT) or driving simulators require the complete integrity of vigilance to achieve normal results. Hypersomnia is usually broadly defined as the combination of abnormally prolonged night-time sleep (regularly 10 h) with EDS during ≥1 months. On the other hand, the term hypersomnia has also been used in a narrower scene for the isolated abnormality of a prolonged night-time sleep need (10 h). “Tiredness”, also in colloquial language often used for sleepiness, in a broader sense also describes the feeling of lack of energy, motivation and initiative. These patients seek rest rather than sleep. They often cannot fall asleep when given the opportunity in spite of feeling tired, and hence, in an MSLT, do not show an abnormally short sleep latency. Furthermore, tiredness (and fatigue) as opposed to sleepiness has a mental (“central”) and physiological (bodily or “peripheral”) component, which the patients can readily distinguish. Patients with insomnia, mild sleep apnea syndrome, or depression rather suffer from mental tiredness than sleepiness during the day. The simple subjective self-assessment using the Epworth Sleepiness Scale (ESS) quite reliably differentiates between sleepiness and mental tiredness (without sleepiness), which makes it a widely used test. The term “fatigue” is also heterogeneously used. In physiology the “fatigue” implied a “time on task performance decrement” to describe decreasing muscle force during a sustained physical effort. In clinical medicine one distinguishes physical (“peripheral”) from mental (“central”) fatigue and the term usually denotes a chronic and more abnormal situation than tiredness. In a broad sense “fatigue” implies a deficiency in coping satisfactorily with mental and physical work load. The chronic fatigue syndrome entails both mental as well as a physical fatigue (so called “leaden paralysis” of limbs). Depressive states are often associated with insomnia and fatigue, but there are also cases with hypersomnia rather than insomnia ( non organic hypersomnia , “atypical depression” or “hypersomnolent depression”) Sometimes these patients have a tendency to spend much of the day lying in the bed without actually sleeping (so called clinophilia). The basic and clinical aspects of fatigu