Infradian Rhythms: What They Are and Why They Matter

Infradian rhythms, like the menstrual cycle, are biological cycles that impact your health
Woman relaxing by a menstrual calendar.

Your body carries out natural, biological cycles every day — and many of these cycles happen without you even paying much attention to them.  

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Perhaps the most commonly known biological cycle is the circadian rhythm, your body’s 24-hour internal clock. Throughout the day, your hormones, organs and bodily systems undergo several changes. From the time you wake up until the time you go to sleep, your internal biological clock impacts your mood, your ability to digest food, your internal temperatures and many other physical, mental and emotional processes. 

But you also have biological cycles known as infradian rhythms. An infradian rhythm is a biological cycle that occurs for longer than a 24-hour period.  

We see infradian rhythms in nature all the time. From hibernation and migration patterns to the molting (or shedding) of skin and mating behaviors, these infradian rhythms contain various phases and often repeat themselves over longer periods.  

In humans, a prime example of an infradian rhythm is the menstrual cycle (and you could even consider seasonal affective disorder as an infradian rhythm, too). To better understand how infradian rhythms evolve over the course of one cycle, Ob/Gyn Erin Higgins, MD, walks us through the various phases of the menstrual cycle, including what impacts your ability to regulate it and how it may change over time. 

Phases of the menstrual cycle 

The menstrual cycle prepares people with a uterus for a possible pregnancy. On average, it’s a 28-day process that’s defined by several phases based on communication between your brain, ovaries and uterus. And what happens during each phase is governed largely by the release or suppression of various hormones that trigger each phase to occur in your reproductive system and transition from one phase into the next. 

Follicular phase 

The follicular phase usually lasts 14 days and is measured from the first day of bleeding, or menstruation (what’s often referred to as the beginning of your period). A few things happen during this phase: 

  • If you’re not pregnant, your uterus sheds its lining (known as menses), which exits your body through your vagina for an average of three to five days
  • At the same time, your ovaries start to produce 15 to 20 eggs inside small sacs called follicles. Over the course of 14 days, one of these follicles becomes dominant and forms a fully mature egg (ovum), while the other follicles are reabsorbed into your body. 
  • The dominant follicle releases more estrogen into your body, which causes your uterine lining to thicken again and prepare for the arrival of the mature egg.

Ovulatory phase 

Around day 14 of your menstrual cycle, ovulation begins. In response to a sudden surge of luteinizing hormone from your pituitary gland, the dominant follicle releases the fully mature egg. This happens so the egg can begin traveling down the super highway that’s your fallopian tube (the connection that runs from your ovaries to your uterus) in the hopes it will find sperm and become fertilized within 24 hours. 

Luteal phase 

This phase takes place right after ovulation and lasts for about 14 days. During the luteal phase: 

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  • The now-empty dominant follicle develops into your corpus luteum, which produces estrogen and progesterone hormones to further thicken and prepare your uterus for a possible pregnancy. 
  • You become pregnant if the egg becomes fertilized as it’s traveling down your fallopian tube and embeds itself into your uterine wall. 
  • If the egg isn’t fertilized, it will pass through your uterus and be expelled with the shedding of the uterine lining for the start of a new menstrual cycle.

Ovulation prediction kits and pregnancy dating are all based on a perfect 28-day cycle,” says Dr. Higgins. “But very few people have a perfect 28-day cycle every single month.” 

What can impact changes in your menstrual cycle? 

Not every menstrual cycle is the same, and even your own menstrual cycle may change from month to month or year to year. Some phases of the menstrual cycle may last longer than average. Some people may experience periods with a heavier flow. And others may not experience menstruation at all (amenorrhea) or have irregular cycles that don’t always line up (oligomenorrhea). 

“There are so many different things that can affect the menstrual cycle,” notes Dr. Higgins. “Your period is not going to be exactly the same month to month and that’s OK.” 

Here are some internal and external factors that can have an impact on the menstrual cycle:

Metabolism and diet 

Your body needs enough energy to undertake all the processes that are involved in the menstrual cycle, including ovulation. If someone is underweight or participates in extreme strength training or exercise, they may stop having their period. This happens when your metabolism slows down because your body is trying to conserve its energy rather than use it for ovulation. 

“If you’ve experienced significant weight loss or there’s disordered eating, your body will recognize that this is not a good environment or situation where pregnancy would be a good thing,” explains Dr. Higgins. “Your body is basically saying that it can’t sustain a pregnancy.” 

The same is true if someone is overweight. Their body fat can increase their level of estrogen and throw off the hormones needed to maintain a normal menstrual cycle. 

Hormone regulation 

As you can see, hormone regulation is key at every step of the menstrual cycle. Think of each hormone as a light switch that, when turned on, forces other switches to turn off. As estrogen increases, progesterone decreases, and vice-versa, so striking the right balance between these hormones is important for maintaining a healthy menstrual cycle and (when necessary) sustaining a pregnancy. Birth control and other medical conditions, like polycystic ovary syndrome (PCOS), also impact the menstrual cycle and can change your pattern of bleeding.

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Stress and sleep 

When you’re stressed or experience disruptions in your sleep schedule, you may experience an increase in cortisol levels. Known as your body’s main fight-or-flight hormone, cortisol helps your body do a number of things that include regulating your metabolism, stress response, blood pressure and blood sugar levels. High cortisol levels can, therefore, cause irregular patterns in a menstrual cycle so that it becomes less frequent or stops completely. 

Other factors that impact your menstrual cycle 

There are several other factors that can impact a menstrual cycle. Your menstrual cycle will change as you age. The length of your periods may fluctuate. And the timing of your periods may change, too. Many of these changes are perfectly normal, as all biological cycles (including infradian rhythms) ebb and flow. But consistent changes can be something to be concerned about, as they may signify underlying conditions that could be treated or managed. 

“We get concerned when cycles are either persistently short or long or if there are other changes such as spotting between cycles or heavy bleeding,” says Dr. Higgins. “That’s when I encourage people to reach out to their doctors because there are so many factors that can be at play.” 

Dr. Higgins suggests keeping a menstrual calendar to track symptoms like the amount of flow, cramping, bowel changes, mood swings and other details to bring on hand when visiting the doctor for regular check-ups.

“Keeping track of changes over the course of three to six months can help us get a bigger picture,” says Dr. Higgins. “Just because something happens once doesn’t necessarily mean there’s something wrong. But knowing your patterns or any persistence of abnormality will be informative when you see your doctor.” 

The more informed you are of your body’s changes over time, including its infradian rhythms and other biological cycles, the more prepared you can be in case new conditions surface and the better you can make the right decisions for your own course of care and treatment.

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