Introduction: Under controlled laboratory conditions, chronic sleep restriction of 3 to 6 hrs/night for 7-14 nights gradually degrades Psychomotor Vigilance Task (PVT) performance. Less is known about whether this accumulative decrement occurs outside the laboratory. US medical residents work 24- to 30-hr extended shifts twice per week, resulting in both acute and chronic sleep loss. Here, we address the impact of working repeated extended duration work shifts on PVT performance. Methods: Seventeen PGY-1 medicine residents (26-32y, 7F) worked 30-hour shifts twice per week in an intensive care unit (ICU) for three weeks, such that they worked at least six extended duration shifts; this followed 3 weeks of non-extended shifts. Subjects completed 10-min PVTs 3-6 times at regular intervals across each extended duration shift. Mean log reaction time (RT) and transformed lapses greater than 500ms (square root of vn+ square root of v(n+1)) were assessed for change over time (extended shifts block 1-6). Controlling for time of day, we assessed differences in performance at the start and end of each shift as a function of the number of extended shifts. Results: Mean RT and lapses worsened with increasing number of extended shifts (one-way repeated measures ANOVA, p<0.05, p<0.02, respectively), with post hoc comparisons showing performance to be worse during the last two extended shifts as compared to the first two (p<0.008). For overnight performance (23:00-08:00h), there was a significant effect of extended shift number for mean RT (p<0.05) but not lapses. For day-time performance (7:00h-14:00h), both mean RT and lapses showed a significant worsening with increasing number of extended shifts (p<0.009, p<0.003, respectively) and time on shift (p<0.002, p<0.0005, respectively). Conclusion: Residents working traditional on-call schedules exhibit poorer performance in the second half of each 24-30-hr extended shift as a result of acute sleep deprivation, and show a chronic deterioration in performance with increasing number of extended shifts.