The recent iiicrcase iii the iiicideiice of shiftivork has beeit accoiiipaiiied by a cltaiige in the type o f task typically perfonlied by the shiftivorker. The tecliiiological advarices nhich have prodiiced both these efficts have iiieaiit that the shifttrorker is iriore likely to be engaged iii cognitive. iiieiitally taxing tasks tliari the predoiiiiiiaiitly perceptiial-iiiotor ones typical of earlier times. Coiitertiporary research oii time o f day effects in perfonnairce efficieiicy has iiidicated that rliese task chaiiges may be crticially iitiportarit iii detenrtirrbig orr-shift per-forinatice, arid lieiice the clioice o f the iiidividiial arid shift system that is ittost siiitable. Evideiice is revieived oti task deitiaiids. tlie e f fc ts of differelit shift systeiiis, and tlie role o f iridividrial differences. A descriptive iiiodel is proposed iii wliicli on-shift perforinaiice is seeti to be depeiideiit oii the type of task, type o f shift systeiti. arid type of person, uvitli the three factors iiiteractiiig via the worker’s variotcs circadian rhythms.