A MULTIDIMENSIONAL CONCEPT OF SLEEPINESS HAS BEEN THE TOPIC OF SEVERAL PUBLICATIONS WITH THE GOAL OF BETTER ASSESSMENT OF THE CONDITION OF EXCESSIVE daytime sleepiness.1-8 A concept of sleepiness as a function of homeostatic and circadian processes

A MULTIDIMENSIONAL CONCEPT OF SLEEPINESS HAS BEEN THE TOPIC OF SEVERAL PUBLICATIONS WITH THE GOAL OF BETTER ASSESSMENT OF THE CONDITION OF EXCESSIVE daytime sleepiness.1-8 A concept of sleepiness as a function of homeostatic and circadian processes was proposed more than 20 years ago by Borbély;9 models proposed since then have stressed the competing drives of wake and sleep8,10 and have included consideration of situational and trait or exogenous factors.6,8 Although the relative importance of the components of sleepiness is somewhat controversial, the complexity of sleepiness and the difficult issues in defining and measuring specific aspects are widely acknowledged. The multidimensionality of sleepiness is supported by many studies showing poor correlations of sleepiness assessed with different measurement tools,11-24 as well as the lack of consistent associations of predictors of sleepiness, such as sleep apnea,18,25-27 with different measures of sleepiness. Most research on the dimensionality of sleepiness has focused on contrasting the specificity of objective measures of the tendency to fall asleep and of the failure to maintain wakefulness.11-13 Some patient-based14-22 and experimental28-31 studies have also shown poor agreement between objective and subjective measures. For example, in a study of sleep-deprived volunteers,28,29 the propensity to fall asleep in nap trials (eg, Multiple Sleep Latency Test [MSLT]) was not strongly correlated with subjective sleepiness, suggesting that measures of subjective sleepiness reflect something other than sleep drive.6,8 Less is known, however, about whether tools that rely on self-report, ranging from questions on feelings of sleepiness and fatigue to subjective estimates of sleepy behavior, are able to measure distinct aspects of sleepiness. A better understanding of subjective measures of sleepiness is particularly important because objective measurement is not feasible in most population research and clinical assessment, particularly in primary care. In the few studies conducted on this topic, only weak correlations between feelings of sleepiness and self-estimated sleepy behavior have been reported,16,23,24 suggesting there are distinct components of subjective sleepiness.32 Alternatively, subjective sleepiness may be inherently unidimensional. Responses to any type of question on sleepiness may predominantly stem from the overarching belief an individual holds regarding their sleepiness. This view, whereby behavior, expectations, and perceptions are integrated, is supported by studies that have found no evidence of multiple factors emerging from analyses of a wide range of sleepiness questions specific to adult33,34 and pediatric35,36 populations. Subjective Daytime Sleepiness: Dimensions and Correlates in the General Population