The menstrual cycle has four phases: menstruation, the follicular phase, ovulation, and the luteal phase. In adults, a healthy cycle typically lasts 21 to 34 days, though cycles up to 45 days are normal for adolescents. Each phase is driven by shifts in key hormones that affect far more than your reproductive system, influencing your mood, energy, sleep, and even the consistency of your cervical mucus.
Menstruation: Days 1 Through 7
Your period marks day one of the cycle. It starts when progesterone levels drop sharply, signaling your uterus that no pregnancy has occurred. Without progesterone’s support, the thickened uterine lining breaks down. Blood vessels in the lining become fragile and permeable, immune cells flood the tissue, and enzymes dissolve the structural scaffolding holding everything together. The result is the mix of blood, tissue, and fluid you see during your period.
Most periods last seven days or fewer. Flow that requires you to change a pad or tampon every one to two hours is considered excessive, especially if it continues beyond a week. During the first few days, many people experience cramping as the uterus contracts to shed its lining. Energy and mood can feel lower during this phase, partly because estrogen and progesterone are both at their lowest point in the cycle.
The Follicular Phase: Building Up Again
The follicular phase overlaps with menstruation, starting on day one and lasting roughly 14 to 21 days. It’s named for what’s happening inside your ovaries: your pituitary gland releases follicle-stimulating hormone (FSH), which triggers your ovaries to develop several small fluid-filled sacs called follicles. Each follicle contains an immature egg.
Over the course of this phase, one follicle becomes dominant and starts producing increasing amounts of estrogen. That rising estrogen does two things simultaneously. In your uterus, it rebuilds and thickens the lining that was just shed. Throughout the rest of your body, it boosts serotonin and dopamine, the chemicals that regulate mood, motivation, and emotional stability. Many people notice improved mental clarity, higher energy, and a more positive mood as the follicular phase progresses, and estrogen is a big reason why.
The follicular phase is the most variable part of the cycle. If your period comes earlier or later than expected, it’s usually because this phase was shorter or longer than usual. The luteal phase, by contrast, stays relatively fixed.
Ovulation: A Brief Fertile Window
Ovulation is the shortest phase, lasting roughly 12 to 24 hours. It typically occurs between day 12 and day 16 of a 28-day cycle, though the exact timing shifts if your cycle is longer or shorter. When estrogen from the dominant follicle reaches a critical threshold, your pituitary gland responds with a surge of luteinizing hormone (LH). That LH surge begins about 36 hours before ovulation and lasts around 24 hours. The egg is then released from the ovary 8 to 20 hours after LH peaks.
If you’re trying to track ovulation, your body offers a few observable clues. In the days leading up to it, cervical mucus shifts from thick, white, and dry to clear, slippery, and stretchy, often compared to raw egg whites. Estrogen drives this change, creating a texture that helps sperm travel more easily. After ovulation, mucus returns to its thicker, drier state as estrogen drops and progesterone takes over. Over-the-counter ovulation predictor kits work by detecting that LH surge in your urine, giving you a 24-to-48-hour heads-up before the egg is released.
The Luteal Phase: The Progesterone Phase
After ovulation, the now-empty follicle transforms into a temporary structure called the corpus luteum. Its primary job is producing progesterone, with some estrogen alongside it. Progesterone changes the uterine lining from simply thick to actively receptive, creating the kind of environment where a fertilized egg could implant and develop. The luteal phase lasts about 14 days in most people, which is why it’s the more predictable half of the cycle.
Progesterone has a calming effect on the nervous system. It increases a neurotransmitter called GABA, which promotes sleep and eases anxiety. Some people feel more relaxed and sleepy during this phase, while others experience the less pleasant side: bloating, breast tenderness, food cravings, and mood changes. These premenstrual symptoms intensify in the final days of the luteal phase as both progesterone and estrogen begin to fall. If the egg wasn’t fertilized, the corpus luteum breaks down, progesterone drops sharply, and the cycle resets with menstruation.
If pregnancy does occur, the corpus luteum keeps producing progesterone for roughly the first 10 weeks, sustaining the pregnancy until the placenta takes over hormone production.
How Hormones Affect Your Day-to-Day
Understanding the hormonal arc across your cycle helps explain patterns you might already notice. Estrogen, which climbs steadily through the follicular phase and peaks just before ovulation, supports memory, mood, and mental sharpness. The shift to progesterone dominance in the luteal phase can make you feel calmer but also more fatigued, and it’s the withdrawal of progesterone at the very end that triggers both your period and the mood dip many people feel in the day or two before bleeding starts.
These aren’t just subjective feelings. Estrogen directly increases serotonin and dopamine production, while progesterone works on entirely different brain pathways, promoting GABA activity that favors rest over alertness. Knowing where you are in your cycle can help you make sense of shifts in your sleep quality, exercise tolerance, appetite, and emotional state rather than treating them as random.
What Counts as Irregular
Cycle length varies from person to person, and some variation month to month is normal. Current clinical guidelines define irregular cycles with specific thresholds based on age. For those one to three years after their first period, cycles shorter than 21 days or longer than 45 days are considered irregular. For adults past that window and before perimenopause, the range narrows: cycles shorter than 21 days or longer than 35 days, or fewer than eight cycles per year, meet the clinical definition of irregular. Any single cycle lasting longer than 90 days at any age after the first year of menstruation is also flagged.
Irregular cycles can reflect a wide range of causes, from stress and weight changes to thyroid conditions and polycystic ovary syndrome. Tracking your cycle length over several months gives you a personal baseline that makes it much easier to spot when something has genuinely shifted versus normal month-to-month variation.