For those endeavoring to develop better methods of measuring/quantifying sleepiness, the "Holy Grail" is a measure that is maximally objective, completely unobtrusive, exquisitely sensitive, and absolutely specific (i.e., varies only as a function of sleepiness). By these criteria, physiological measures (e.g., based on brain activity such as EEG, fMRI, near-infrared spectroscopy, etc.) would appear to hold the most promise. However, from an operational standpoint, the utility of a sleepiness measure is derived not from its ability to sensitively reflect the brain's extant level of sleepiness per se, but from the implications that this level of sleepiness has for the individual's current and near-term ability to safely and efficiently perform operationally-relevant tasks. Thus, an ideal operationally-relevant sleepiness measure is one that is unobtrusively embedded in the actual operational task, and allows sleepiness-related performance deficits to be distinguished from performance deficits due to other causes. Toward this end, we have developed a PVT-derived metric that incorporates the entire distribution of responses within a PVT session, and reflects changes in the pattern of performance that can be used to identify and quantify "state instability"--the putative physiological state that specifically underlies sleepiness-induced performance deficits.