I don't pay close enough attention to the timing of my bleeding to notice minor alterations, and I haven't experienced many major disruptions. Rumour has it that all sorts of things can pre-empt or realign a womanly cycle, including stress, fatigue, illness and the like.
The only ones I have noticed are induced bleeding due to jet travel (always pack menstrual gear when travelling. I don't know why planes do this to women, but moving more than a couple of time zones will always make me bleed within a few days), and girl to girl influences.
Research quoted at Museum of Menstruation suggests that females' cycles will align themselves so that they ovulate and bleed at about the same time, using some sort of chemical signal that seems to be transferred by touch. As far as I know people still aren't sure about these particular pheromones being detected and transmitted by smell. This might explain the common idea that women's cycles only align if they are friends (friends presumably touch each other).
I personally have had my cycle warped while living in a household of ladies, and my two femme roommates at the time also complained of unpredictable bleeding (my own periods got slightly shorter and more irregular) and bloating at irregular moments. We lived together for 8 months, and by the halfway mark all our periods were starting within a week of each other.
After that house, I lived with some boys in another city, then moved back to town into a house containing 2 boys, one of which is special, and one of the femme roommates from before. I was on the pill at this point, so my cycle was tethered, but the femme roommate had a terrible bout of suddenly unpredictable bleeding (which she was unused to, being 20 years old and fairly settled in her ways) and weird cycle lengths, until after about 6 months she lined up with me, give or take about 4 days. She claims that this experience was worse than what she noticed at our old house; I would venture that this was because she wasn't meeting me at the middle of our cycle difference, but rather compensating completely for it without me changing at all.
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