Thomas MJW, Ferguson SA. Prior sleep, prior wake, and crew performance during normal flight operations. Aviat Space Environ Med 2010; 81:665–70. Introduction: Industries that operate outside daytime hours are known to carry higher safety risks related to fatigue. While we are beginning to understand better the role of fatigue in increasing the risk of accidents in the workplace, relatively little is known about the manifestation of fatigue in the multicrew environment, where operational safety involves interaction between two or more crewmembers and a complex operating environment. Method: Data were collected by trained expert observers during 302 normal flight operations of a commercial airline flying short-haul jet operations. Crewmembers were asked to provide an estimate of their total sleep in the prior 24 h, total sleep in the prior 48 h, and total wake time since their last sleep period at the commencement of cruise. Observers used the Threat and Error Management Model, developed as a standardized and highly structured method to collect operational performance data. Results: Restricted sleep in both the 24-h and 48-h period prior to each sector were found to be associated with changes in crews’ threat and error management performance. However, prior wake was not associated with any significant changes in crew performance. Restriction to less than 6 h sleep in the prior 24 h was associated with degraded operational performance and increased error rates. Discussion: The findings of this study provide support to the notion that prior sleep is a critical fatigue-related variable. Moreover, the use of individual subjective assessment of prior sleep as a component of an overall fatigue risk management system is reinforced.