Sleep and the menstrual cycle affect each other in both directions, which is what makes the problem feel circular.
Sustained stress raises cortisol, and persistently high cortisol can suppress the signals that trigger ovulation. That is why cycles lengthen or vanish during periods of real strain — illness, grief, over-training, undereating.
The cycle pushes back. Body temperature rises slightly in the luteal phase, which fragments sleep for some women, and falling progesterone before a period disturbs it further.
The practical opening is usually sleep, because it is the most modifiable: consistent timing matters more than duration, and light exposure in the morning does more than most things taken at night.
Cycles that stay absent for three months or more deserve investigation rather than patience.