Most women expect perimenopause to announce itself with hot flushes. For a large proportion, the first thing that actually changes is the calendar, years earlier, and it goes unremarked because a slightly shorter cycle is not a symptom, it is an inconvenience.
What is happening physically
You were born with a finite number of ovarian follicles. As the pool declines, the hormonal signal to recruit them rises, so recruitment happens faster. The follicular phase, the stretch between the first day of bleeding and ovulation, contracts. The luteal phase after ovulation stays roughly the same length, so the whole cycle shortens from the front.
A cycle that was reliably twenty-eight days becomes twenty-five, then twenty-three, and then, later in the transition, starts skipping entirely and stretching out to forty and sixty days. Shortening first, lengthening later, is the usual arc.
Why this is worth tracking
Cycle length is the only part of this that is objectively measurable at home, without a blood test, without a clinician, and without argument. A woman who arrives at an appointment able to say “my cycles have gone from twenty-eight to twenty-three days consistently over the last eight months” is having a different conversation from one who says she thinks something has changed.
Write it down. It is the cheapest leverage available in this entire process.