The present study was done to determine whether weight gain was more prevalent in workers on late shifts than in those on day shifts. A questionnaire about changes in weight, food intake, exercise, and sleep since starting the job on the current shift was given to day-shift and late-shift (evening and night) hospital workers. Data were analyzed for 85 subjects, 36 of whom worked during the day shift and 49 the late shift. The late-shift group reported a mean weight gain of 4.3 kg, which was greater than the mean weight gain of 0.9 kg for the day-shift group (P = 0.02). There were, however, no significant differences in current body mass index (26.7 +/- 5.4 SD) between groups. There was a trend for late-shift workers to report eating more since beginning the later shift (P = 0.06). When combined with those reporting exercising less (P = NS), this trend became significant (P = 0.04). Late-shift workers reported eating fewer meals (1.9 +/- 0.9 SD) than the day-shift workers (2.5 +/- 0.9; P = 0.002). In addition, late-shift workers reported eating the last daily meal later (mean = 22:27, or 10:27 PM) than day-shift workers (17:52 or 5:52 PM; P < 0.00005). Late-shift workers also reported more naps (P = 0.01) and longer naps (P = 0.05) during the work week than did day-shift workers. The reported changes in eating, exercise, and sleep may contribute to the increased weight gain of late-shift workers.