Controversies in the measurement of daytime sleepiness.

The measurement of sleepiness is crucial to the the circadian rhythm as part of our understanding work of sleep researchers and sleep disorder cliniof “sleepiness”. cians. This topic is critically important for sleep Often, the definition of sleepiness has been cirdisorder clinicians in order to make decisions recular: sleepiness is a state that occurs in individuals garding diagnosis and treatment for their patients who have had inadequate sleep. This tautological who are at increased risk for sleepiness-related approach to defining “sleepiness” occurs because accidents. Researchers trying to understand the of our lack of knowledge regarding the precise complex relation between sleep and wakefulness physiological substrates of sleep. With a thorough also require an unassailable measure of sleepiness. understanding of the physiology of sleep, we would Many different approaches to the quantification of not need to rely on an abstract construct and could sleepiness have been proposed over the last several instead measure directly the process of sleep. Once decades, and there is debate within the scientific these processes are well identified, sleepiness might community as to the “best” methodology for this be defined according to specific neurophysiological purpose. processes that are linked with sleep (or lack of The task of scientifically understanding sleepiness sleep). requires clarity at two levels: (1) the definition of A limitation in research in this area is that “sleepthe theoretical construct and (2) the operational iness” is not usually defined explicitly as a theoretical definition of that construct. A definition of “sleepconstruct. Operational definitions for the measureiness” as a construct has evolved along with the ment of “sleepiness” are provided, but “sleepiness” growth in the field of sleep research. When looking itself is presented as if it has just one definition. across studies, it becomes clear that, instead of just This is further complicated because the term “sleepone definition, various theoretical constructs are iness” has a common usage and therefore has many implied by different investigators. Although the term connotations that may not reflect its meaning in a “sleepiness” is used in each case, there are difscientific context. Also, the term “sleepiness” is ferences in the theoretical construct according to often used interchangeably with “tiredness” and the experimental question under investigation. For “fatigue” by the public or governmental agencies. example, studies from the 1970s were likely to view Use of the term “sleepiness”, therefore, seems sleepiness as a physiological state that occurs as a to imply a specific definition even though various result of increased homeostatic drive for sleep. researchers have different constructs in mind. The Since then, research in biological rhythms has exsecond problem is that many different operational panded this view to include impairment in waking definitions may be used to measure sleepiness, even function that fluctuates according to the phase of when the construct is being defined in the same way. Researchers who are studying homeostatic sleep drive may use mean sleep latency from the Multiple Sleep Latency Test (MSLT), or may use a self-report measure such as the Epworth Sleepiness Phone: 312-942-5440, Fax: 312-942-8961, E-mail: estepans