Analysis of the time of day of onset of menstruation.

The exact time of day of onset of menstruation was studied in 86 women over 4-6 cycles. Detailed information on the time of retiring the previous night, time of rising in the morning, time of onset of premenstrual spotting and time of onset of any menstrual cramps was obtained from 48 of these women. A significantly greater number of cycles (70.4%) commenced during the night or in the first 4 h after rising, compared with later in the day. In a large proportion of these (29 out of 76), blood was noted to be present on waking, menstruation thus having begun at some time during the hours of sleep. Premenstrual spotting of greater than 3 h duration was more common than expected and was noted by 67% of women. Onset of menstrual cramps was fairly evenly distributed closely either side of commencement of menstruation proper in the 50% of women who experienced them. In only 13% of women studied did bleeding commence within the same 3-hour time interval in each of four cycles, and almost 50% demonstrated a very wide variation with onset at almost any time of day.