Persistent fatigue usually has an identifiable cause, and it’s rarely just “not getting enough sleep.” The list of possibilities ranges from nutritional deficiencies and hormonal imbalances to sleep disorders you may not know you have, disrupted body clocks, and lingering effects from past infections. Understanding the most common culprits can help you figure out which ones apply to you and what to do next.
Iron Deficiency
Iron deficiency is one of the most common and most overlooked causes of constant tiredness, especially in women of reproductive age. Your body needs iron to make hemoglobin, the protein in red blood cells that carries oxygen to your tissues. When iron drops, less oxygen reaches your muscles and brain, and the result feels like dragging through the day no matter how much rest you get.
Iron deficiency is defined by a ferritin level (the protein that stores iron inside your cells) below 30 nanograms per milliliter. Severe deficiency falls at 15 or below. The tricky part is that your hemoglobin can still test in the “normal” range even when your iron stores are depleted, so a standard blood count alone won’t always catch it. If fatigue is your main complaint, ask specifically for a ferritin test. Heavy periods, plant-based diets without careful planning, frequent blood donation, and digestive conditions that limit absorption are all common reasons ferritin drops without you realizing it.
Thyroid Problems
Your thyroid gland sets the pace for nearly every metabolic process in your body. When it underperforms, a condition called hypothyroidism, everything slows down: your energy, your digestion, your ability to stay warm, even your mood. Fatigue from hypothyroidism tends to feel heavy and unrelenting, often paired with weight gain, dry skin, and brain fog.
Diagnosis involves a blood test measuring thyroid-stimulating hormone (TSH) along with the actual thyroid hormones T3 and T4. There’s also a milder version called subclinical hypothyroidism, where TSH is slightly elevated but T3 and T4 still look normal. Even this mild form can produce noticeable fatigue in some people. Hypothyroidism is more common in women, in people over 60, and in those with a family history of autoimmune disease.
Sleep Apnea
You can spend eight or nine hours in bed and still wake up exhausted if your breathing repeatedly stops during the night. Obstructive sleep apnea causes the tissues in your throat to collapse during sleep, blocking airflow for seconds at a time, sometimes dozens or even hundreds of times per hour. Each interruption jolts your brain just enough to restart breathing but not enough to wake you fully, so you may have no idea it’s happening.
Sleep apnea is diagnosed when a sleep study records five or more breathing interruptions per hour, and treatment is typically recommended at 15 or more per hour. But here’s what’s interesting: research shows that the severity score alone doesn’t predict how tired someone feels. Patient-reported symptoms, like unrefreshing sleep, morning headaches, and daytime drowsiness, matter just as much as the numbers. If your partner says you snore loudly, gasp during sleep, or stop breathing, those are strong clues. Being overweight increases risk significantly, but thin people get sleep apnea too, particularly those with a naturally narrow airway or a recessed jaw.
Blood Sugar Swings
If your fatigue hits hardest an hour or two after meals, blood sugar fluctuations may be the issue. Reactive hypoglycemia occurs when blood sugar drops too low within about four hours of eating. The pattern usually goes like this: you eat something high in refined carbohydrates, your body overproduces insulin in response, and your blood sugar crashes below where it started. The crash brings weakness, fatigue, shakiness, and difficulty concentrating.
Even without a formal diagnosis, plenty of people experience milder versions of this cycle. Meals heavy in white bread, sugary drinks, or processed snacks tend to spike blood sugar fast and drop it fast. Meals with protein, fat, and fiber slow the whole process down, keeping energy more stable. If the post-meal slump is your primary pattern, adjusting what you eat (and eating at more regular intervals) is often enough to notice a real difference within days.
Disrupted Circadian Rhythm
Your body runs on an internal clock that governs when you feel alert and when you feel sleepy, and that clock depends heavily on cortisol, a hormone that normally surges in the morning and tapers off at night. A healthy cortisol awakening response produces a 50 to 150 percent rise in cortisol within 30 to 45 minutes of waking up. That surge is essentially your body’s natural caffeine. When it’s blunted, meaning it rises less than 50 percent, you feel groggy and flat all morning.
Shift work is a well-documented cause. Night-shift workers consistently show a delayed and flattened cortisol pattern, with a weakened morning peak and an inability to properly suppress cortisol at night. Over time, their bodies fail to fully adapt to the altered schedule, contributing to chronic fatigue and elevated stress. But you don’t have to work nights to disrupt your rhythm. Irregular sleep times, staying up late on screens, and inconsistent wake-up times on weekdays versus weekends all have similar effects on a smaller scale.
Blue light from phones and tablets before bed is a specific culprit. Research shows that evening exposure to blue light without a filter elevates morning cortisol levels while simultaneously reducing the cortisol awakening response, essentially scrambling the signal your body uses to transition between sleep and alertness. If you scroll your phone in bed and then feel unrested in the morning, this is a likely mechanism.
Post-Viral Fatigue
If your fatigue started after a viral illness, even one you thought was minor, you may be dealing with post-viral syndrome. This condition can follow infections like COVID-19, the flu, mononucleosis (Epstein-Barr virus), shingles, and several other common viruses. A diagnosis of post-viral fatigue applies once symptoms have persisted for at least two weeks after the infection itself has cleared.
Most people recover within a few weeks to a couple of months, but if symptoms last beyond six months, the condition may meet the criteria for chronic fatigue syndrome (ME/CFS). If you still feel wiped out three weeks or more after a viral illness, that’s a reasonable point to seek evaluation rather than waiting it out.
Chronic Fatigue Syndrome (ME/CFS)
ME/CFS is a distinct medical condition, not just “being really tired.” The CDC’s diagnostic criteria require a substantial reduction in your ability to function at pre-illness levels lasting more than six months, accompanied by fatigue that is new (not lifelong), not caused by unusual exertion, and not relieved by rest. Two other symptoms must also be present: post-exertional malaise and unrefreshing sleep.
Post-exertional malaise is the hallmark that separates ME/CFS from ordinary fatigue. It means that physical, mental, or even emotional effort that would have been manageable before now triggers a crash, typically 12 to 48 hours after the activity, that can last days or weeks. Even sensory overload from noise or bright light can set it off. Unrefreshing sleep means a full night of sleep leaves you feeling just as tired as when you went to bed, even when no specific sleep disorder is found on testing.
At least one additional symptom is also required for diagnosis: either cognitive impairment (problems with memory, concentration, and processing speed that worsen with exertion) or orthostatic intolerance (symptoms like lightheadedness, increased fatigue, and nausea that get worse when you stand or sit upright and improve when you lie down). These symptoms need to be present at moderate or greater intensity at least half the time.
Other Common Contributors
Several other conditions frequently show up as fatigue without many other obvious symptoms. Depression often manifests as physical exhaustion before emotional symptoms become apparent. Vitamin D deficiency, vitamin B12 deficiency, and dehydration are common and easily tested for. Medications including antihistamines, blood pressure drugs, and antidepressants can cause fatigue as a side effect. Type 2 diabetes and prediabetes produce fatigue through chronically elevated blood sugar that impairs how efficiently your cells use energy.
Red Flags Worth Knowing
Most causes of persistent fatigue are manageable once identified, but certain combinations of symptoms warrant prompt evaluation. Unexplained weight loss alongside fatigue can point to an overactive thyroid, diabetes, liver disease, or in some cases cancer. A persistent or high fever combined with fatigue and other symptoms like a lingering cough needs medical attention right away. Night sweats, swollen lymph nodes that don’t resolve, or fatigue that’s worsening steadily over weeks rather than staying stable are also signs that something beyond lifestyle factors may be at play.