A year after having her second baby, Jen Novak, 38, was sleeping seven to eight hours a night. Yet she would wake with a “halo of exhaustion” that hovered over her all day. The Twin Cities–based ad-agency account director felt foggy, slow, and not herself.
As a college student, Novak battled Graves’ disease, a form of hyperthyroidism that brings feelings of jitteriness and anxiety. It showed her how easily hormones can wreak havoc when they’re out of balance. Now, postpartum, she suspected the pendulum might have swung the other way.
“When you’re a parent of young children, people say, ‘Of course you’re tired,’ but I knew something else was going on in my body,” Novak says. “I’m sleeping. I’m eating well. I’m doing all the things, but I’m just constantly exhausted.”
Novak isn’t alone. Globally, about one in five adults experiences general fatigue, and one in 10 experiences chronic fatigue lasting more than six months, according to a 2023 systematic review and meta-analysis published in Frontiers in Public Health.
“Fatigue is one of the most frequent complaints we hear from patients,” says functional-medicine physician Gregory Plotnikoff, MD, MTS, FACP. “It’s such a significant problem in our society, and there are a million different reasons for it.”
Fatigue is a lack of energy. Our cells use oxygen and nutrients to produce energy in the form of adenosine triphosphate (ATP) inside their mitochondria. Signals from hormones and the autonomic nervous system (ANS) influence that process. Anything that limits the supply of nutrients and oxygen, disrupts hormones or ANS signals, or diverts resources to respond to chronic stress or a depleted immune system can sap our energy.
In other words, persistent exhaustion is a single symptom with a wide variety of potential root causes.
“Fatigue is a generic sign from the body, saying, ‘Hey, something’s not OK,’” says functional nutritionist Jesse Haas, CNS, LN.
For that reason, fatigue rarely shows up in isolation. Studying the pattern of symptoms that accompany it can often help point to its underlying cause or causes.
“Fatigue is the tip of a very large iceberg,” Plotnikoff notes. “Drilling down to the base can require a lot of time and attention.”
Several other factors complicate the challenge: Basic lab tests often miss critical dysfunctions; multiple causes may overlap; and mental health factors such as burnout and grief pull hard on the same systems.
But most root causes of fatigue can be managed effectively once you’ve identified them, and restoring energy can have a big impact on overall well-being.
Learn about some of the most common drivers of fatigue — and how to address them.
Thyroid Dysregulation
The thyroid, a butterfly-shaped gland in the neck, makes hormones that regulate the body’s metabolism, controlling how efficiently it converts nutrients and oxygen into energy.
Some of these hormones, particularly T3 and T4, affect how quickly the body burns calories at rest. When the thyroid produces optimal levels of these hormones, body temperature and heart rate increase, and the nervous system activates to produce quicker reflexes and greater alertness. But when the thyroid grows sluggish, so do we.
An underactive thyroid — a condition known as hypothyroidism — can leave you “feeling like you’re carrying around bags of cement,” says Haas. You might also feel cold, dry, constipated, and foggy.
“Thyroid [production] has to be very low for prolonged periods of time for a lot of these symptoms to appear,” notes Plotnikoff. This means that low-grade thyroid problems may be sapping stamina well before the classic signs emerge.
How to Address:
A basic clinical approach often includes testing for thyroid-stimulating hormone (TSH), but it can be worth going deeper with a full thyroid panel, says Haas. “There’s a pretty broad range in which TSH is considered to be normal, but it can still be suboptimal.”
Even if TSH is normal, suboptimal levels of T3 and T4 can cause symptoms.
If testing indicates a sluggish thyroid, a combination of treatment and lifestyle adjustments can help get things back on track: targeted thyroid medication when appropriate plus steady, protein-anchored meals; stress and sleep support; and follow-up labs to make sure the complete thyroid picture (not just TSH) is improving.
“These are easy things to test and relatively easy to correct when they’re out of whack,” Haas says.
Hormonal Imbalance
Like thyroid hormones, cortisol (the “stress hormone”), insulin, and various sex hormones affect our energy production and metabolism.
Some of these — such as estrogen, DHEA-S, insulin, and testosterone — are anabolic, or what Plotnikoff calls “build-up hormones.” They contribute to tissue growth and repair and support increases in energy-creating mitochondria.
Others — such as cortisol and adrenaline — are catabolic, or “break-down hormones” that disassemble molecules and release stored energy.
“Imagine a teeter-totter, with the build-up hormones on one side and the break-down hormones on the other,” Plotnikoff says. “You want to have a good balance between them. If the break-down side is heavier than the build-up, then people experience fatigue.”
Other symptoms that might point to imbalanced hormones are shifts in weight, loss of muscle and bone, changes to mood and cognition, disrupted sleep, delayed injury recovery, and low libido.
Haas notes that hormonal transitions, such as puberty, pregnancy, and perimenopause, often come with fatigue. “These are times when there’s a lot of brain pruning going on — a clearing out of stuff we don’t need anymore before we start rebuilding.”
How to address:
To some extent, fatigue is a natural byproduct of this physiological remodeling, so Haas suggests we offer ourselves and others plenty of patience and rest during these transitions.
Minimizing stress — whether environmental, physical, emotional, or dietary — is also key to hormone balancing, says Plotnikoff. So is stabilizing blood sugar with regular, nutrient-dense meals, which can reduce a reliance on cortisol spikes for energy.
Targeted supplements or hormone replacement therapy can provide additional support.
Blood-Sugar Dysregulation
Your cells run best on a steady glucose supply, which is supported by regular, well-balanced meals. Skipping meals or waiting too long to eat can trigger crankiness and brain fog. Snacking on high-sugar foods or refined carbs can trigger big insulin spikes, followed by crashes that leave you lethargic.
Over time, blood-sugar swings can also disrupt sleep, mood, and hormonal balance. Once you’ve added stress hormones (which raise glucose) and poor sleep (which reduces insulin sensitivity), you’ve got a recipe for persistent fatigue.
How to address:
“One of the things I recommend for patients with low energy or fatigue is establishing a superconsistent eating routine that’s balanced in protein, fat, and fiber,” says Haas.
If you’re nourishing your energy needs throughout the day, she adds, your blood-sugar levels should feel more like a mellow kiddie-park roller coaster than an extreme thrill ride.
If you suspect you have trouble regulating your blood-sugar levels, a few straightforward labs — including fasting glucose, A1c, and fasting insulin — can offer some insights. And a continuous glucose monitor can provide a window into how your body responds to specific foods or meal timing.
Other lifestyle shifts, such as taking a walk after meals, prioritizing sleep, managing stress, and engaging in strength training, can help stabilize blood-sugar levels, too. You might also try taking magnesium or berberine supplements.
Sleep Apnea
Sleep apnea — a condition in which breathing repeatedly stalls during sleep — is another hidden driver of daytime fatigue. These pauses jar the brain awake to reopen the airway, disrupting sleep and preventing deep rest.
Sleep apnea can also cause blood-oxygen levels to drop, straining the heart and brain. This can lead to drowsiness and difficulty concentrating.
Studies have estimated that between 10 and 30 percent of U.S. adults suffer from sleep apnea. And yet it can be difficult to know if you have it, especially if you live alone and there’s no one to tell you if you’re snoring — one of sleep apnea’s hallmark signs. Even then, while certain health conditions may predispose a risk for sleep apnea, not all of them are visible.
“When I was in med school decades ago, I was taught that sleep apnea is a problem for people who are obese or elderly,” Plotnikoff says, noting that it’s often missed in those who don’t fit that profile. “At our clinic, we recently diagnosed it in a 17-year-old slender person, whose only risk factors had to do with the anatomy of their throat and hypermobility.”
Signs of sleep apnea can include unrefreshing sleep, loud snoring, headaches, and brain fog. FDA-approved portable devices measure airflow, oxygen levels, respiratory effort, and heart rate to help identify the condition. An in-person overnight sleep study is even more thorough.
How to address:
Many sufferers find it helpful to limit their drinking, because alcohol can relax the throat muscles and increase airway obstruction. For heavier people, weight loss may help; weight around the neck can put pressure on the airways.
CPAP (continuous positive airway pressure) machines are among the most commonly prescribed treatment tools for sleep apnea. They help keep the airway open during sleep and can substantially improve sleep quality.
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Nutrient Deficiencies
Inadequate nutrition is another potential cause of fatigue. When our cells don’t get the raw materials they need to function, energy output drops. Common deficiencies include iron, vitamins B12 and B9, vitamin D, and magnesium.
Iron deficiencies are a routine culprit. Hemoglobin is the iron-containing protein in our red blood cells that carries oxygen to our organs and tissues; low levels can be a sign of iron-deficiency anemia. “Anyone who menstruates or donates blood should be getting their hemoglobin levels tested,” Haas says.
Testing ferritin, a protein that stores iron, is another way to measure iron levels. “Fatigue is a really common symptom of suboptimal ferritin,” she adds. “There’s a huge range of ‘normal’ ferritin levels in conventional medicine, but it’s better for people who are menstruating regularly to have higher levels.”
How to address:
Good sources of iron include oysters, red meat, and poultry. Pumpkin seeds, lentils, and dried apricots are also iron-rich. (Learn how to get more iron from your diet at “How to Make Sure You’re Consuming Enough Dietary Iron.”)
Meanwhile, vitamins B12 and B9 are necessary for red-blood-cell formation; low levels indicate poor oxygen delivery and sluggish energy production. Low vitamin D levels can also sap vitality. And sufficient magnesium, which many of us lack, is key to making and using ATP.
Even if these nutrients aren’t technically deficient, suboptimal levels can still show up as fatigue, says Haas. Notable hair loss, slow-healing wounds, bone pain, and an irregular heartbeat are other good reasons to get nutrient levels checked.
Working with a practitioner to adjust your diet and then target areas for supplementation is usually the most effective way to address deficiencies.
Stealth Infection
Hidden, low-grade infections can quietly siphon energy. These include viral reactivations (such as EBV, the virus that causes mononucleosis), smoldering sinus or dental infections, postviral states, tick-borne illnesses, or gut or urinary infections that don’t clearly announce themselves. Any of them can lead to a persistently “on” immune system that keeps you feeling foggy and underpowered.
“Chronic infections can keep your body in a chronic state of low-level alarm,” writes integrative-medicine physician Aviva Romm, MD, in The Adrenal Thyroid Revolution. She notes that chronic stress can also reduce your body’s ability to keep viruses contained.
“We carry a lot of viruses with us, whether we’ve had chicken pox, mono, or other things,” says Plotnikoff. Normally, the body sequesters these viruses, but they may reemerge in times of stress — such as when a latent herpes virus awakens to trigger a cold sore as the body is fighting off a cold.
How to address:
In general, managing stress can help the immune system do its job. A quality diet also helps. “Focus on nutrient density,” Haas advises. “High-fiber foods, adequate protein, anti-inflammatory herbs and spices, and lots of colorful foods to support the microbiome.”
If you suspect a potential infection may be at the root of your fatigue, see a healthcare provider. They can help you dig deeper with targeted testing and treatment.
Autonomic Nervous System Dysfunction
The ANS runs our background settings, including heart rate, blood pressure, breathing, body temperature, and digestion, so we don’t have to think about them. When the ANS malfunctions, it may lead to symptoms of dysautonomia, in which ordinary activities like standing or taking a hot shower become strangely depleting.
“Right now, there’s an epidemic of dysfunction in the ANS,” Plotnikoff notes. “We have more and more people with impaired capacity to literally stand up.”
The ANS has two primary branches: the sympathetic, also known as “fight or flight,” and the parasympathetic, usually called “rest and digest.” (It also encompasses the enteric nervous system in the gut.) Communication between the sympathetic and parasympathetic systems helps manage blood flow to the brain.
“Right now, there’s an epidemic of dysfunction in the ANS,” Plotnikoff notes. “We have more and more people with impaired capacity to literally stand up.”
When the balance between the two is off, blood can pool in the legs, and the heart races in an effort to get enough blood to the brain. “If you don’t get good brain perfusion, you’re going to be fatigued; you’re going to feel weak; you’re going to feel like you need to lie down,” Plotnikoff says.
When people with ANS dysfunction do lie down to rest, their brains get adequate blood flow, and suddenly they feel wired and unable to sleep. Adds Plotnikoff, “This is a very common thing that drives a multibillion-dollar-a-year insomnia industry.”
Signs of ANS dysfunction can include lightheadedness upon standing, graying vision that clears upon sitting, a racing heart, breathlessness, brain fog, or the odd combo of feeling tired but wired. Testing can include electrocardiograms, physical or neurological exams, and tilt-table tests that measure how different body positions affect heart rate.
How to address:
Fixing nervous-system imbalances that lead to poor blood flow to the brain can often effectively take care of symptoms, Plotnikoff says. Tools might include diet changes, exercise, or medication. “Suddenly, people’s brain fog is gone, and their clarity, capacity, and energy are significantly improved.”
Burnout, Grief, and Depression
Fatigue often feels distinctly physical — and it is. But the mind, spirit, and emotions can still be driving causes. Long periods of overwork, significant personal loss, and mood disorders can all change how the brain allocates energy.
In burnout, physical and emotional resources become deeply depleted by chronic stress. It can feel as if our body and emotions have gone on strike, refusing to keep up the breakneck pace. “We don’t thrive when we’re constantly in survival mode,” says Haas. “Sometimes fatigue is the body saying, ‘Hey, dude, I can’t do what you’re asking of me. We need less.’”
Fatigue is also a common element of grief. Feelings like sadness and anger consume substantial emotional energy, and after a big loss, the mind has to remap a world that’s changed — which is a lot of work. Expecting too much from ourselves too soon, before our energy stores have had time to replenish, might lead to prolonged feelings of depletion.
Depression comes with its own challenges, including imbalanced neurotransmitters, increased inflammation, and disrupted sleep patterns. All these are treatable, but motivation to do so can be hard to find.
How to address:
“Exercise is really important in [treating] depression, but when you’re really fatigued, it’s hard to do any exercise,” Plotnikoff notes. “Part of the goal is to break that spiral and reverse it upward so people are getting dopamine hits from small achievements.” (Try these five tips to start moving if you’re struggling with your mental health.)
In cases like these, the basics of self-care — stabilizing sleep, moving the body, eating wholesome and nourishing food — provide an important foundation. Maintaining reasonable expectations and celebrating the small wins can help; so can patience and self-compassion.
Finally, working with a trusted care provider can be extremely supportive when energy is brought low by life’s challenges. You don’t have to do it all alone.
Chronic Fatigue Syndrome
This article originally appeared as “Fatigue, Explained” in the May/June 2026 issue of Experience Life.
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