Changes in the Heart Rate During Sleep With Menstrual Cycle Under Free-Living Conditions

In this study, we observed the changes in the heart rate (HR) during sleep with respect to the menstrual cycle under free-living conditions. Seven female university students with normal menstrual cycles were allowed to live freely for one month without controlling their sleeping and waking times. The HR was measured by a wristwatch wearable device during nighttime sleep. Considering the circadian rhythm, the time of the lowest point of the circadian rhythm (min CR time) was estimated using HR during sleep, and the change in HR at min CR time (min HR) with menstrual cycle was evaluated. For comparison, the sublingual temperature was measured immediately after waking up as basal body temperature (BBT). The three subjects whose biological rhythms were disturbed showed no significant difference in BBT between the follicular and luteal phases. On the other hand, regarding min HR, all subjects showed a significant difference between the follicular and luteal phases (P<0.05), and it increased in the luteal phase compared to the follicular phase. Thus, the HR at the lowest point of the circadian rhythm (min HR) may be an important parameter for monitoring the menstrual cycle in women with irregular rhythms.