Dozens of factors can shift the timing, flow, and regularity of your period, from everyday habits like sleep and exercise to underlying health conditions. A normal menstrual cycle ranges from 21 to 35 days, and variation within that window is expected. But when your cycle suddenly changes or falls outside that range, something is usually driving it. Here’s what can throw your period off and why.
Body Weight and Body Fat
Your body needs a certain threshold of body fat to maintain a regular cycle. This is the same principle behind why girls need to reach a minimum weight before their first period arrives. When body fat drops too low, the brain reduces its signal to the ovaries, and ovulation stops. The exact cutoff varies from person to person. Some women can be quite lean and still menstruate normally, while others lose their period at a body fat level that wouldn’t seem particularly low.
On the other end of the spectrum, carrying excess weight can also disrupt your cycle. Higher body fat increases the amount of estrogen circulating in your body, which can lead to heavier, longer, or less predictable periods. Significant weight gain or loss in a short timeframe is one of the most common triggers for a missed or late period.
Exercise Volume and Intensity
Physical activity is healthy for your cycle up to a point. Beyond that point, it becomes a stressor your body responds to by dialing down reproduction. A large study of over 3,700 women found that training at low intensity for seven or more hours per week, or moderate intensity for six or more hours per week, was associated with roughly 43 to 46 percent higher odds of missing or infrequent periods compared to training two to three hours per week. High-intensity training carried similar risk at five or more hours per week.
The mechanism isn’t fully understood. It likely involves a combination of low energy availability (burning more calories than you take in), elevated stress hormones, and reduced body fat. What matters most isn’t the exercise itself but whether your body has enough fuel left over to support its normal hormonal rhythms.
Stress and Sleep Disruption
Psychological stress triggers a rise in cortisol, a hormone that can suppress the brain’s signal to start ovulation. A demanding few weeks at work, a family crisis, or chronic anxiety can all delay your period or cause you to skip one entirely. The effect is usually temporary, resolving once the stressor passes.
Sleep disruption works through a related pathway. Your reproductive hormones follow a circadian rhythm, and when that rhythm is thrown off, your cycle often follows. Shift workers are significantly more likely to experience irregular periods and painful menstruation. One study found that 53 percent of premenopausal women working shifts reported changes in menstrual function, compared to about 20 percent of women in the general population. Travel across multiple time zones can produce a similar, though usually shorter-lived, effect.
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common hormonal conditions in women of reproductive age, and irregular periods are its hallmark. The core problem often starts with insulin resistance, where the body’s cells don’t respond well to insulin, so the body produces more of it. That excess insulin drives the ovaries to produce too much testosterone and other androgens.
High androgen levels inside the ovaries cause a cascade of problems. They trigger the recruitment of many small follicles at once, but none of them matures enough to release an egg. Without ovulation, the uterine lining builds up longer than usual, leading to skipped periods followed by unusually heavy ones when bleeding finally occurs. The more severe the insulin resistance, the worse the menstrual disruption tends to be.
Thyroid Disorders
Both an overactive and underactive thyroid can change your period, though they do it differently. An overactive thyroid (hyperthyroidism) most commonly causes lighter, less frequent periods. About one in five women with hyperthyroidism experience some type of menstrual change, though most continue to ovulate normally.
An underactive thyroid (hypothyroidism) also tends to cause less frequent periods, but severe cases can stop ovulation altogether. Mild hypothyroidism may not affect your cycle at all, and ovulation and even conception can still occur. If your periods have changed and you can’t explain why, thyroid function is one of the first things worth checking with a blood test.
Hormonal Birth Control
Hormonal contraceptives are designed to alter your cycle, so changes while using them are expected rather than alarming. The pill often makes periods lighter and more predictable. Hormonal IUDs can reduce bleeding dramatically, and some women stop having periods altogether. Injection-based contraceptives commonly cause irregular spotting, especially in the first few months, followed by lighter or absent periods over time.
Starting, stopping, or switching between methods can all cause temporary irregularity. After stopping hormonal birth control, it can take a few months for your natural cycle to re-establish itself. This is normal and doesn’t indicate a fertility problem.
Other Medications
Several non-hormonal medications can change your period as a side effect. Blood thinners, including aspirin, can make periods heavier because they reduce your blood’s ability to clot. Common pain relievers like ibuprofen and naproxen can have the opposite effect, sometimes reducing flow. Chemotherapy drugs frequently disrupt or stop periods, sometimes temporarily and sometimes permanently depending on age and treatment duration. Thyroid medications can also shift your cycle, particularly while the dose is being adjusted.
Certain psychiatric medications, especially antipsychotics, can raise levels of a hormone called prolactin, which suppresses ovulation and can cause missed periods. If your cycle changed around the time you started a new medication, that connection is worth mentioning to your prescriber.
Perimenopause
Cycle changes are one of the earliest signs that the transition to menopause has begun. Most women notice shifts in their 40s, but some see changes as early as their 30s. Perimenopause is driven by fluctuating and gradually declining estrogen and progesterone levels, which make ovulation less predictable.
In practical terms, this means your periods may come closer together for a while, then further apart. Flow can swing from light to unusually heavy from one month to the next. You may skip periods entirely, then have one return. This phase typically lasts several years before periods stop for good. A gap of 60 days or more between periods is common as you get closer to menopause.
Nutrition and Iron Deficiency
Restrictive eating, crash dieting, and nutritional deficiencies can all suppress ovulation in much the same way that low body fat does. Your reproductive system is energy-expensive to run, and when calories or key nutrients are scarce, the body deprioritizes it.
Iron deficiency deserves a specific mention, though it works in the opposite direction. Heavy periods are a leading cause of iron deficiency in women, not the other way around. About one in five women have heavy periods, putting them at especially high risk. The relationship can become a cycle of its own: heavy bleeding depletes iron stores, and severe anemia can worsen fatigue and other symptoms that make the situation harder to manage. If your periods are consistently heavy, checking your iron levels can catch a deficiency before it becomes serious.
Early Cycles in Adolescence
If you’re a teenager and your period seems unpredictable, that’s largely normal. It can take up to three years after your first period for cycles to become regular. During this time, the hormonal feedback loop between the brain and ovaries is still maturing, and longer cycles, skipped months, and heavier or lighter flow are all common. Consistency typically improves with time.