The investigation of the efficacy and safety of drugs requires assessments of their effects on alertness/sleepiness. Unfortunately, there is confusion about the nature of ‘sleepi-ness’, the factors which influence it, and how it can be measured under different circumstanc-es. This review aims to clarify these matters and to offer some suggestions about how current difficulties might overcome. Different meanings of the word ‘sleepiness’ are examined ini-tially. Methods that purport to measure ‘sleepiness’ are then examined, including their test-retest reliability and the relationship between the results of different measurements within the same subjects. Some objective methods are found not to be as reliable as was initially report-ed. Information about the reliability of several other methods is either inadequate or non-existent. One assumption which underlies two frequently used objective methods for measur-ing ‘sleepiness’ (the Multiple Sleep Latency Test and the Maintenance of Wakefulness Test) is that the ‘sleepier’ a person is, the quicker they will fall asleep. While this assumption has face validity, other assumptions about these tests are re-examined and are found wanting, at least sometimes. The difficulty arises in part because it is not always clear when the sleep on-set process begins and ends. ‘Sleepiness’ is found to be influenced much more by short-term factors, such as the subject’s posture at the time and during the preceding few minutes, than has been acknowledged previously. Some possible solutions to these difficulties are suggest-ed, including a new conceptual model of sleep-wake control, with implications for the design of drug trials.
Keywords: sleepiness, alertness, drowsiness, sleep propensity, sleep-wake models, drug trials
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