A study was conducted on the levels of performance and alertness of employees using two different workshift schedules. Workers at a natural gas utility worked 8 or 12 hour rotating day and night shift schedules and levels of performance and alertness were measured using computerized tests as well as self reports. Those on the 12 hour shift schedule had significantly better scores on two computerized measures of performance and reported feeling less sleepy than those on the 8 hour schedule. Significantly more frequent simple reaction time misses and hand tremors were seen during the 12 hour shifts. Those on the 12 hour shifts reported higher subjective workloads compared with the 8 hour shift workers. Multiple regression analyses indicated that there were circadian fluctuations in performance and subjective state and most measures showed declines at night. The worst performance, with the exception of digit addition, was seen using the 12 hour shift schedule. Daily sleep and habit shift scores demonstrated few similarities between the repeated measures and between groups analyses of variance of average shift scores. A decline in total sleep time was seen across the work week of 12 hour shifts. The authors recommend caution when scheduling important activities during extended workshifts, especially on extended nightshifts.