Biologically, the menstrual cycle is divided into four phases: the follicular phase, ovulation, the luteal phase, and menstruation. Corresponding to these phases, there are changes in the levels of pituitary hormones (FSH and LH) and in the levels of ovarian hormones (estrogen and progesterone), as well as changes in the ovaries and the uterus. A fairly common menstrual problem is dysmenorrhea, or painful menstruation.

Research indicates that some, though not all, women experience changes in mood over the phases of the menstrual cycle. For those who experience such changes, their mood is generally positive around the middle of the cycle (i.e., around ovulation), whereas negative moods characterized by depression and irritability are more likely just before and during menstruation. These negative moods and physical discomforts are termed the premenstrual syndrome. On the other hand, research indicates that there are no fluctuations in performance over the cycle. There is evidence suggesting that fluctuations in mood are related to changes in hormone levels, but data also suggest that mood fluctuations are related to cultural factors. Research attempting to document whether men experience monthly biological and/or psychological cycles is now in progress.

The climacteric is the period in middle age during which the functioning of the ovaries (both hormone and egg production) declines gradually. One symptom of this process is menopause, the cessation of menstruation. Physical symptoms, such as hot flashes, during this period result from declining levels of estrogen and may be relieved by hormone-replacement therapy; contrary to popular belief, research does not show an increased incidence of depression at the time of menopause. Changes in sexual functioning across the menstrual cycle and at menopause are most likely related to changes in levels of testosterone.

Men experience a much more gradual decline in the functioning of their gonads. Despite popular beliefs, only a small minority of men experience a midlife crisis.