Women turn in earlier than men because their circadian rhythms are faster than men’s. (Circadian rhythm refers to the daily rhythmic activity cycle, based on 24-hour intervals.) That’s one of the possible explanations why they may be too tired for sex in the late evenings. Researchers at Harvard University have found that, on average, the cycle length of the biological clock in women is shorter as compared to men’s. Also, women’s circadian rhythms run six minutes faster than men’s, making them much more likely to be early birds rather than night owls.