Sleepiness as a state-trait phenomenon, comprising both a sleep drive and a wake drive

In this paper it is proposed to extend the classical models of sleepiness in two ways. Firstly, the role of a wake drive, besides a sleep drive, in determining sleepiness is emphasised. Although this has already been suggested in literature and convincing arguments can be found, it is not generally accepted. Secondly, we argue to incorporate trait aspects of sleepiness or a long-term person-specific level of sleepiness, besides short-term changes of sleepiness or 'state-sleepiness'. Shortly, a conceptualisation of sleepiness in which situational wake drive and sleep drive modify a basal level of both wake drive and sleep drive, is proposed. It implies that sleepiness can result from essentially different factors involved: a chronic condition or an acute state of either a high level of sleep drive, a low level of arousal or a combination of both. This is illustrated in a working model and potential assessment tools for the wake drive and state versus trait sleepiness are discussed. (C) 2003 Elsevier Science Ltd. All rights reserved.