This study investigated whether the temporal stress of working at night interferes with nurses' experience of different phases of the menstrual cycle Twenty-four, rotatmg-shift nurses recorded data on a hand-held computer at the start and end of each day and every 2 hours during their shifts for a period of 28 days The measures included a sleep diary, self-ratings of mood and symptoms, and two cognitive performance tasks Using a pooled time series analysis, it was found that some of the self-report measures showed a significant interaction between type of shift worked and menstrual cycle phase For example, alertness was found to be lower towards the end of the night shift during the premenstrual phase compared to other phase and shift combinations However, some of the measures showed only main effects of cycle phase or were nonsignificant Retrospective measures from a larger sample showed an association between the number of nights worked per year and duration of premenstrual and menstrual problems, and seventy of premenstrual problems The results suggest that nightwork may alter some aspects of nurses' psychological experience of the menstrual cycle