The end result is much like that for obesity, in that there is either irregular bleeding or no bleeding at all. There seems to be a critical weight below which the hormones won't function normally. In medical school we were taught it was 106. But practicing obstetrics for twenty years has seen many under-100 not only cycle normally but become pregnant without difficulty. A woman who is underweight and also exercises to an extreme will carry a double-whammy in menstrual irregularities.
Drugs can alter the cycle as well.
Now I'm the first one to say I'll take my air conditioned home with city services over cave-living, but the further we get away from our true circadian and natural lives, the more our internal systems have to adjust to the new world orders. But the reproduction is a deal-breaker if we want to survive. We can reset the thermostat with some flexibility, but there are limits beyond which the system won't work.