When your period starts, estrogen and progesterone are at their lowest point in the entire menstrual cycle. This sharp drop is what actually triggers menstrual bleeding, and it sets off a cascade of changes across your body, from your immune system to your mood to your metabolism. Understanding what’s happening hormonally during those days can help make sense of how you feel physically and emotionally.
Why Bleeding Starts: The Hormone Drop
The entire period is driven by one event: the breakdown of a small, temporary structure on your ovary called the corpus luteum. After you ovulate each month, this structure forms from the empty follicle and pumps out progesterone and estrogen. Its job is to keep the uterine lining thick and nutrient-rich in case a fertilized egg needs to implant. If pregnancy doesn’t happen, the corpus luteum has a built-in expiration date of about 14 days.
The breakdown happens in two stages. First, progesterone production drops off. Then the structure itself physically shrinks, eventually becoming a small piece of scar tissue on the ovary. Without progesterone and estrogen holding it in place, the top layers of your uterine lining lose their blood supply and begin to shed. That shedding is your period. A typical period lasts about 4 to 5 days, with total blood loss of roughly 2 to 3 tablespoons.
By day 1 of your period, progesterone levels have fallen to between 0.1 and 0.7 ng/mL. For context, during the second half of your cycle (when the corpus luteum is active), progesterone can be 10 to 20 times higher. Estrogen is similarly low. These are the lowest hormone levels you’ll experience all month.
How Your Body Restarts the Cycle
That hormone crash isn’t just an ending. It’s also a signal for the next cycle to begin. When estrogen and progesterone drop low enough, your brain’s pituitary gland responds by releasing more follicle-stimulating hormone, or FSH. This rise in FSH starts during your period, even while you’re still bleeding. FSH does exactly what its name suggests: it stimulates small follicles on one of your ovaries to start growing. Between days 6 and 14 of your cycle, one of those follicles will become dominant and begin producing its own estrogen, which gradually rises and eventually stops the bleeding by thickening the uterine lining again.
So your period isn’t a pause in hormonal activity. It’s the transition point where one hormonal wave ends and the next one begins, overlapping right in the middle of menstruation.
Mood Changes and Serotonin
If you feel more irritable, flat, or emotionally sensitive around your period, the hormonal shift offers a direct explanation. Estrogen and progesterone both influence serotonin, one of the brain chemicals most closely tied to mood regulation. When those hormones drop before and during menstruation, serotonin activity can dip as well.
Research from the Max Planck Institute found that just before menstruation begins, a protein that clears serotonin from the gaps between brain cells becomes more active. That means serotonin gets pulled away faster than usual, leaving less of it available to stabilize your mood. For most people, this produces mild emotional shifts. For the roughly 3 to 8 percent of menstruating people who have premenstrual dysphoric disorder (PMDD), the brain appears to be hypersensitive to these normal hormonal fluctuations, producing symptoms severe enough to interfere with daily life.
These mood changes typically peak in the day or two before bleeding starts and improve within the first few days of the period, as the brain adjusts to the new, stable low-hormone baseline.
Your Immune System Shifts Gears
Hormones don’t just affect your reproductive system. They also tune your immune response throughout the month. During the second half of your cycle, when progesterone is high, your immune system leans toward a more tolerant, anti-inflammatory state. This makes biological sense: if an embryo were implanting, an aggressive immune response could attack it.
When progesterone and estrogen crash at the start of your period, your immune system pivots. It becomes more pro-inflammatory, favoring the type of immune activity that fights infections and clears damaged tissue. This is partly why some people notice that autoimmune symptoms like joint pain or skin flares can worsen around their period, while others find they’re slightly better at fighting off colds during menstruation. The immune shift also helps your body efficiently break down and clear the shedding uterine lining.
Body Temperature Drops
If you’ve ever tracked your basal body temperature, you’ve seen this hormonal shift in real time. After ovulation, progesterone raises your resting body temperature by roughly 0.5 to 1.0°F. Your temperature stays elevated throughout the second half of your cycle. When progesterone drops just before your period, your temperature falls back to its lower baseline. This drop typically happens a day or two before bleeding begins and is one of the earliest physical signs that your period is on its way.
Insulin and Metabolism
Your body’s ability to manage blood sugar also fluctuates with your hormones. A large longitudinal study tracking women across full menstrual cycles found that insulin resistance is lowest during menstruation, with a score of 1.3 on a standard measure. After ovulation, when estrogen and progesterone rise, insulin resistance climbs to around 1.59. Both estrogen and progesterone were positively associated with insulin resistance in the study data.
In practical terms, this means your body handles carbohydrates and sugar most efficiently during and just after your period. The cravings many people experience before their period may partly reflect the body working harder to manage blood sugar when insulin resistance is higher. Once menstruation starts and hormones bottom out, that metabolic stress eases. Some people notice they feel less bloated and have more stable energy during their period compared to the days leading up to it, and this insulin shift is one reason why.
What You’re Feeling Is Real
The combination of these changes explains why menstruation can feel like more than just bleeding. Low estrogen reduces serotonin availability, which can flatten your mood. Low progesterone removes its mild sedative effect, which may disrupt sleep. The immune shift toward inflammation can cause mild achiness or fatigue. Prostaglandins released to help the uterus contract and shed its lining are the same compounds that cause cramps, and they can also trigger nausea or loose stools when they enter the bloodstream.
By around day 3 to 5, estrogen begins its slow climb again as a new follicle takes over on the ovary. Many people notice a corresponding lift in energy and mood during the tail end of their period as this hormonal recovery begins. The low point is brief, and the body is already building toward the next ovulation before the bleeding fully stops.