ABSTRACT
This paper concerns four aspects of the daily variations encountered both in daytime alertness/drowsiness levels and in so-called psychomotor vigilance in narcolepsycataplexy. They are: (i) the physiological changes used as measures of alertness/ drowsiness; (ii) the issue of whether or not qualitatively different forms of sleepiness and drowsiness exist; (iii) the effects of the variations in alertness/drowsiness upon sensitive performance tasks, in particular on so-called vigilance tests; and (iv) the underlying mechanism(s) of drowsiness in narcolepsy. The word drowsiness is generally employed rather than sleepiness, as it denotes an objective physiological state, whereas sleepiness has a much broader and less precise meaning.
