Menstrual cycle phase is a potential confound in psychophysiology research.

Effects of menstrual cycle phase on physiological levels and reactivity were studied in a within-subject design with 20 women aged 18-30 years. Basal body temperature was monitored daily; follicular and luteal recording sessions occurred before and after the ovulatory temperature rise, respectively. The design consisted of task and rest periods, presented in counterbalanced order across phases. Half the subjects began during their follicular phase and half began during their luteal phase. Follicular and luteal phases were compared for differences between baseline resting levels, as well as for reactivity. The results showed menstrual phase differences in heart rate, pulse transit time, and EEG alpha, particularly during tasks, but no difference in reactivity. Habituation effects in the group that began during the follicular phase often masked luteal activation. In spite of the fact that not all statistical results achieved significance, the consistency of most trends supports the conclusion that physiological recording sessions with women might best be restricted to the follicular phase.