This review considers the relationship between sleep need and sleepiness. In healthy adults, objective measures of sleepiness (e.g. Multiple Sleep Latency Test; Psychomotor Vigilance Test) and subjective indices (e.g. Stanford Sleepiness Scale) often poorly inter-correlate and have been seen as orthogonal dimensions. This is perhaps not surprising as the methodology of these tests is quite different in, for example, their duration, testing environment, whether they are experimenter versus participant-paced, and the understanding and expectancy of participants. It is argued, here, that 'sleepiness', the 'propensity to fall asleep' and the 'need for sleep' are not synonymous, but qualitatively different. They may represent different positions on a dimension ranging from essential to non-essential sleep/sleepiness, and the position on this dimension is detected to varying extents by the different measures. As these tests can detect - and perhaps induce - levels of sleepiness which would be undetectable by, and of little concern to people under everyday situations, they can reveal a sleepiness having the potential to be misinterpreted as sleep debt. © 2009 Elsevier Ltd. All rights reserved.
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