Neurobehavioral performance and alterations in mood consequent to sleep disorders and their treatment has recently been the focus of clinical investigations. Primarily in patients with obstructive sleep apnea (OSA), there has been increased interest in the effects of sleep fragmentation and hypoxemia on the brain's ability to process information and to alter affect. The purpose of the second part of this two-part series is to describe measures that could be applied to document the impact of sleep disorders on neurobehavioral performance and mood, discuss factors affecting the selection of measures for research and practice, and to describe evidence generated by the use of these instruments in research. The neurobehavioral deficits that accompany sleep deprivation can be categorised as decrements in cognitive throughput, working memory and sustained attention. Usually evaluated using tasks of short duration, impairments associated with OSA have included impaired information processing and decline in the total number of completed and/or correct responses per unit time. Using assessments of working memory, including short recall maneuvers involving words or paragraphs, investigators have documented in sleep apnea patients the inability to consolidate and recall material producing deficits in the recollection and retention of new information. Evaluations of sustained attention appraise reaction time, the ability to remain on task, and the number of errors of omission and false responses. Evidence suggests that OSA patients display slowing of response time as well as increased errors, lapses and number of false responses. Similar deficits have been documented with sustained and divided attention tasks that present the respondent with challenges in tracking and reaction response, tasks required for driving. Although untreated sleep apnea patients were more impaired than normal controls, their performance on such tasks was greatly enhanced following CPAP treatment. In addition to substantial clinically meaningful improvements in many areas of neurobehavioral performance, changes in mood have also accompanied treatment for sleep apnea. 2001 Harcourt Publishers Ltd