The human menstrual cycle is roughly the same duration as the lunar month, yet it is not synchronized with a specific phase. The average length of a menstrual cycle is 28 days, but it can be as short as 24 days or as long as 30 days. Menstruation is a biological process that occurs over several periods during a woman's life. The term "menstrual period" refers to the five to seven day interval between menses. Menstruation is a natural process that is essential for removing the blood from the uterus after pregnancy. Menopause is the permanent termination of menstruation due to loss of ovarian function after the age of 45. Based on the idea that time is related to movement, the seasons are believed to be related to the cycles of nature and thus they are called "natural seasons". In reality, however, scientists believe that there is no evidence that links humans to any kind of natural phenomenon that would cause them to give us guidance on when to plant or harvest our food. Rather, the Earth's climate changes constantly, and these changes influence what grows well where at any given time. Scientists think that some ancient ancestors of modern-day people may have lived in a more seasonal environment about 10,000 years ago, but they did not learn to cultivate crops until much later.