The irregular shift system in Finnish hospitals with schedules traditionally planned for short 3-week periods creates problems in the social life of the personnel. Current backward rotation with 'quick returns' and rest periods between the shifts which are too short cause fatigue. The possibilities provided by legislation for individual and functional flexibility in the planning of schedules have not been utilized because of the hierarchical structure of hospital organizations. This study was aimed at finding measures to reduce the strain caused by irregular shift work. It consisted of two parts: a survey of midwives doing a three-shift work in hospitals (n = 366), and an intervention in six maternity wards (n = 45). During the 6-month intervention the influence potential of the personnel increased, and the number of quick returns fell. As a result, the mental strain of the work decreased and the stress levels fell. The positive effects were evident especially among the elderly midwives: their work became less strenous and their social interaction improved. Despite the positive effects of the intervention, 55% preferred the former system with longer continuous free time. The physiological criteria for a good shift system were outweighed by the demands of social life, emphasizing the importance of individual flexibility and participation in the planning process.