Dissecting circadian performance rhythms: implications for shiftwork.

Safety on shift systems will depend, at least in part, on the adjustment of shiftworkers' circadian (c.24h) rhythms in performance capabilities which will in turn depend on their underlying control. In the present study three volunteers lived on a 30h 'day' and performed a range of tasks every 2h (while awake) to allow estimates of the relative magnitude of the endogenous (body clock) and exogenous (masking) components of their rhythms to be made. Performance rhythms were found to differ considerably in this respect with, for example, speed on a 4-choice serial reaction time task appearing to be largely endogenously determined while that on a 5-target Sternberg task was more dependent on exogenous (i.e., 30h) factors. This implies that performance measures may adjust at very different rates to one another to night work, and hence that the optimal form of shift system may vary according to the precise demands of the shiftworker's task.