Background Twelve-hour shifts contribute to flexible patterns of work, but the effects on delivery of direct care and staff fatigue are important topics for deeper examination. Aims To examine the impact and implications of 12-hour shifts on critical care staff. Methods A staged dual approach using two focus groups (n ¼ 16) and questionnaires (n ¼ 147) with critical care staff from three critical care units. Results Positive effects were found with planning and prioritizing care, improved relationships with patients/relatives, good-quality time off work and ease of travelling to work. Less favourable effects were with caring for patients in isolation cubicles and the impact on staff motivation and tiredness. Acceptable patterns of work were suggested for numbers of consecutive shifts. and rest periods between shifts . Conclusions Most participants believed 12-hour shifts should continue. The challenge is to ensure existing systems and practices develop to improve on the less positive effects of working 12-hour shifts. Implications for nursing management This study provides nurse managers with important and relevant staff views on the impact of working 12-hour shifts. In particular to those working within a critical care environment and suggests the challenge is to ensure existing systems and practices develop to improve on the less encouraging effects of working 12-hour shifts. It adds an understanding of the senior nurse’s view on the positive and negative effects of managing and organizing staff off duty to safely run a department with 12-hour shifts.