This study tests the hypothesis that permanent night nurses would be no better off than rotating shift nurses on the first night of a run of duty. duty. Thirty permanent and 30 rotating shift female nurses from the same hospital participated in a study involving sleep diaries, oral temperature measurement as well as subjective ratings of well-being and the effort needed to complete their work. Although the permanent nurses fared significantly better than rotating shift nurses in subjective ratings, this did not appear to be due to the maintenance of a nocturnal orientation in the permanent group. Significant differences between the groups only emerged in the amount of night sleep before the shift. Day sleep amounts were similar between groups, as was the size of the temperature drop (0.5 degree F) between midnight and 4 a.m. Thus, lifestyle and social differences between the groups appeared to be more likely than biological rhythm adjustment ones to account for the permanent group's better subjective ratings.