Humans, unlike other animals do not go through an estrous cycle and are always ready to have sex.
There is no physical seasonal change in men or women, that make them more horny. Of course, since men are more visually oriented, men may feel hornier during the summer, when people where less clothing. This is especially true if you live accross the street from a topless beach.
Women do have some slight degree of differing levels of sexual interest that coincides with the mestrual cycle. During ovulation, (a bit under 2 weeks after the first day of menses) women have a higher level of estrogen and progesterone and are therefore a bit hornier. The evolutionary advantage in this, is that women are only fertile during ovulation, and women who have sex during this time are more likely to conceive.
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