Many, if not most, of the studies of night work and night workers since 1918 have major design flaws which make interpretation of their results difficult and unreliable. Moreover, the absence of specific studies of police night shifts requires that conclusions about such shifts rely on 'expert' opinions and subjective extrapolation. Every contemporary researcher who has studied the effects of night work holds that it should be outlawed or regulated to limit night shifts to no more than two consecutive nights without 48 hours off. Researchers maintain that unless a person maintains sleep patterns during days off, adaptation to night work does not occur. Research indicates that individuals' adaptation to night work depends on individual biological rhythms, sleep patterns, social and environmental rhythms, and work rhythms. Future research should focus on differing individual tolerances for shift work, such that persons assigned night work will be best suited for it. Operational definitions and 85 references.