For both the MSLT and the MWT, there have been no large, multicenter, prospectively collected data to establish normative values, so available data from smaller, more limited studies have been used to describe cutoff values. Methodological differences in these studies, such as different definitions for sleep onset, nap duration, and patient selection, impact the outcomes of these studies. In addition, MSLT data from sleepy subjects demonstrate a “floor effect” (ie, a sleep latency lower than even a very sleepy subject is likely to achieve), and MWT data from healthy subjects may exhibit a “ceiling effect” (ie, many subjects are able to remain awake for the duration of the trial; this effect is reduced in the 40-min MWT) Thus, the data are not normally distributed in healthy subjects. Other difficulties shared by both tests involve adjustments for age, sex, and underlying disease in the interpretation of the data. For example, although both tests are, in practice, used in the evaluation of pediatric patients, there is insufficient evidence to provide age-specific normative data.