ABSTRACT
Fatigue in operational settings has many facets, as is discussed in various other contributions. A number of mathematical models has been developed to predict fatigue and performance under conditions of sleep loss and/or circadian misalignment. The net level of fatigue resulting from these two processes is modulated by a variety of other factors, which are subsumed under the broad categories of internal states and external states. Performance instability in particular makes fatigue problematic in operational settings, as it leads to human error at unpredictable moments in time. The normative way to control work times in order to manage fatigue involves promulgation of hours of service regulations, which typically prescribe maximum durations of duty time and minimum durations of rest. However, because hours of service regulations disregard sleep/wake history and circadian rhythmicity, they may at times be overly restrictive and inflexible, and at other times overly permissive and unsafe.
