Seasonal influence of diurnal rhythms in the onset of the plasma luteinizing hormone surge in women.

The time of the LH surge onset in plasma was detected in 75 human cycles. The preovulatory LH surge began between midnight and 0730 h in two thirds of the patients. According to the season of study, the LH surge began between 1030 and 2215 h in 52.4% of the spring cycles compared with 12.9% of the cycles studied during other seasons (P less than 0.001). The highest frequency of the LH surge occurrence over a 3-h time interval was 1500 +/- 1.5 h during the spring cycles (28.6%) and 0300 +/- 1.5 h (42.5%) during the other seasons. It was estimated that ovulation occurred primarily in the morning during the spring and primarily in the evening during autumn and winter.