Women know that their monthly periods are not always on time, or even monthly. A menstrual cycle lasts between 21 and 35 days, and it's normal to fluctuate a few days here and there, but there are months where you skip your period completely. Going three months without your period is most likely a sign of amenorrhea.

So, why do some women miss their period even when they're regular?

In SciShow's video, “Why Did You Skip a Period?” host Michael Aranda explains while skipping periods can mean you're pregnant, it can also happen when you're breastfeeding, using a certain birth control, or have a medical condition that directly interferes with hormones. However, there are other things that can lead to a missed period, like a lack of energy. Energy is important for keeping the body working properly, including your menstrual cycle.

Read More: 6 Reasons For Late Period After A Negative Pregnancy Test

Typically, the menstrual cycle is kicked off by the hypothalamus, a gland in the brain that releases a hormone called gonadotropin releasing hormone (GRH). This hormone causes the pituitary gland to release two more hormones, estrogen and progesterone, which regulate the ovaries and affect menstruation most directly. Estrogen helps build the uterus lining that would nourish a fertilized egg; if the egg isn't fertilized, it breaks apart, and the lining and the egg shed during menstruation.

This process can be disrupted if there isn't enough energy available, such as exercising too much or too hard. This is why amenorrhea is most common among ballet dancers, gymnasts, and long distance runners. A 2002 Penn State study involving monkeys found women who exercise experience a change in their menstrual cycle. These monkeys were encouraged to boost their food consumption for energy and nutrients. A calorie deficiency can stop periods too. In fact, amenorrhea is one of the diagnostic criteria for anorexia.

Exercise and too few calories both affect the menstrual cycle because they have a similar effect on the body: an energy deficiency. This puts your body in preservation mode, as it focuses on conserving fuel for the most important functions, and reproduction isn't prioritized. The amount of energy you use at rest goes down, and you produce hormones at different levels. This leads to producing less GRH, which means the pituitary gland doesn’t release the hormones that regulate estrogen and progesterone. This means no period.

Read More: How Your Diet Can Influence Your Menstrual Cycle

Too many missed periods could lead to future health problems like osteoporosis. People with amenorrhea tend to have low estrogen levels — estrogen helps maintain bone density — which means that even people who are very young can be at risk of developing osteoporosis if they have amenorrhea.

Adjusting lifestyle factors, like getting adequate nutrient intake, moderate exercise, and living a healthy and balanced life, can help keep periods regular.