Scientists have had a hard time studying the exact mechanisms because stress is such a complex thing, explains James Greene, MD, medical director for women’s health at Kaiser Permanente in Washington. But here's the gist of what’s happening in the body: “We presume there is some suppression in the functioning of the hypothalamus and pituitary gland in the brain [when you’re stressed],” Greene says. “These glands work in tandem to control our endocrine system that includes the ovaries and adrenal glands.” This then influences your ovarian hormone function and ovulation, or the releasing of the egg, he explains. Because it hasn’t been studied as much, doctors still do know how the degree of stress or length of exposure to it causes the period to slow down.