Fertility is the ability of people or animals to produce healthy offspring in abundance, and of the earth to bear fruit. Human fertility depends on factors of nutrition, sexual behavior, culture, instinct, endocrinology, timing, economics, way of life, and emotions. Both women and men have hormonal cycles which determine both when a woman can achieve pregnancy and when a man is most fertile. The female cycle is approximately twenty-eight days long, but the male cycle is variable. Women ovulate at about the fourteenth day of their cycle, this obviously being the most fertile time for females. Men can ejaculate and produce sperm at any time of the month, but their libido dips occasionally, which may be in relation to an internal cycle. Women's fertility peaks in their early twenties, and often deteriorates after 30.