It's called the Dormitory effect/McClintock effect. It's pretty interesting science. It is speculated that chemicals called pheromones secreted from the axilla (armpit) of a woman affects the menstrual cycle of her roommate or other women living with her. (1).... https://twitter.com/thesanasi/status/1265772774494285825

These pheromones stimulate the receptors in the "vomeronasal organ" present within the nasal cavity of her roommates. Impulses from these receptors are transmitted to the hypothalamus which in turn influences the menstrual cycle via the hypothalamo-pituitary-gonadal axis....

This is one thing Sembulingham's Textbook of Medical Physiology taught me that Guyton didn't. I'll still pick Guyton over Sembu anyday though, but when push comes to shove, I'll recommend combining them both with a side dish of Ganong of course . Glad those days are far behind.