Any physical stress, whether it's from excessive exercise, psychological burdens, insufficient calorie intake, or even just travel, can mess with this pathway. In the specific case of excessive exercise, Dunaif says, endogenous opiates (also known as endorphins, which give rise to a "runner's high") might be involved, though more research is sorely needed to know for sure. An increase in endorphins, which happens when you exercise frequently, might directly suppress the hormones involved in this pathway. In fact, she says, some studies have found that if you give athletes with amenorrhea an opiate antagonist like naltrexone, a subset of them will see a return of their menstrual cycles. But not all of them will. Often a loss of menstruation is multifactorial, so all of those stressors may need to be addressed to gain back a period.