error was reported in 9.8% of months that had between one and four extended-duration work shifts and in 16% of months that had five or more extended-duration work shifts. furthermore, the frequency of attentional failures was strongly associated with the frequency of extended-duration work shifts. evidence from this study further corroborates the negative impact that extended-duration work shifts may have on performance, as well as increased accidents and injuries. working extended hours or overnight shifts also poses the added difficulty of requiring performance from an individual at a time with the body is driven to sleep by the circadian system. sleep, alertness, and cognitive functioning are determined by the interaction of two processes: the endogenous circadian pacemaker and the homeostatic drive for sleep. the endogenous circadian pacemaker generates the 24-hour circadian rhythm that regulates subjective alertness and sleep propensity as well as core body temperature, cognitive functions, and melatonin secretion, as described above. it is also highly sensitive to light, which is its primary synchronization. misalignment of the circadian rhythm results in disturbed sleep, impaired performance alertness, waking-hour melatonin secretion, and reduced levels of nocturnal secretion of growth hormone. the outcome, therefore, can range from performance error to long-term health decrements. individuals who work at night and attempt to sleep during the day suffer because the timing of their sleep/wake schedule remains out of phase with the timing of the environmental light. night workers are particularly prone to vehicle accidents, and their decreased alertness, performance, and vigilance are likely to blame for a higher rate of industrial accidents and quality control errors on the job, injuries and a general decline in work productivity rate. recent information also suggests that as the body normally releases melatonin when it is dark, working under artificial like at night suppresses the released of melatonin, which may increase the risk of developing cancer. in summation, ground-based evidence demonstrates that sleep loss, circadian desynchronization, and extended work shifts lead to increased performance errors and accidents. the extent to which these risk factors are also present in the space flight environment is therefore an important consideration.