The results of some intervention studies on the effects of the change from weekly rotating to quicker rotating shift systems are presented. Consequently, the following recommendations for the design of shift systems according to physiological, psychological and social criteria are discussed: (1) Nightwork should be reduced as much as possible. If this is not possible, quickly rotating shift systems are preferable to slowly rotating ones. Permanent nightwork does not seem to be recommendable for the majority of shiftworkers. (2) Extended workdays (9-12 hours) should only be contemplated when the nature of work and the workload are suitable for extended working hours, and the shift system is designed to minimize the accumulation of fatigue and toxic exposure is limited. (3) An early start for the morning shift should be avoided. Flexible working time arrangements can be achieved in all shift systems. The highest flexibility is possible in the so-called 'time autonomous groups'. (4) Quick changeovers (e.g. from night shift to afternoon shift on the same day) should be avoided. The number of consecutive working days should be limited to five-seven. Every shift system should include some free weekends with at least two consecutive days off. (5) The forward rotation (phase delay, clockwise rotation: morning/evening/night shift) would seem to be most preferred.