Constant tiredness usually comes from one of a handful of common causes: poor sleep quality, nutritional deficiencies, an underlying medical condition, or a mental health issue like depression or anxiety. Often it’s a combination. The tricky part is that fatigue is one of the least specific symptoms in medicine, showing up in dozens of conditions, which is why pinpointing the real cause matters more than just pushing through it.
Sleep Problems You Might Not Realize You Have
The most obvious cause of constant tiredness is also the most overlooked, not because people forget about sleep, but because they assume they’re sleeping fine when they’re not. You can spend eight hours in bed and still wake up exhausted if your sleep quality is poor. Sleep apnea is a prime example: your airway partially or fully collapses repeatedly during the night, each time lasting at least 10 seconds, pulling you out of deep sleep without fully waking you. Many people with sleep apnea have no idea it’s happening. Loud snoring, gasping during sleep, and morning headaches are common signs.
Beyond sleep apnea, inconsistent sleep schedules, late-night screen use, alcohol before bed, and caffeine consumed too late in the day all fragment your sleep architecture. You cycle through lighter stages of sleep instead of spending enough time in the deep, restorative phases. The result feels identical to not sleeping enough, even if your total hours look reasonable.
Iron, B12, and Vitamin D Deficiencies
Your body needs iron to make red blood cells that carry oxygen to your tissues. When iron stores drop too low, every cell in your body gets less fuel, and fatigue is usually the first symptom you notice. The World Health Organization considers ferritin (the protein that stores iron) below 30 µg/L in children and below 70 µg/L in adults a sign of iron deficiency when inflammation is present. Menstruating women, vegetarians, and people with digestive conditions that reduce absorption are at highest risk.
Vitamin B12 deficiency causes a similar kind of fatigue, along with numbness in the fingers and memory problems. It’s especially common in people over 50 (who absorb less B12 from food), vegans, and anyone taking long-term acid-reducing medications. Low vitamin D is another frequent contributor. Because these deficiencies develop gradually, you may not connect the slow creep of exhaustion to a nutritional gap. A simple blood panel can check all three.
Thyroid Disorders
Your thyroid gland controls your metabolic rate, essentially how fast your body burns energy. When it underperforms (hypothyroidism), everything slows down: you feel cold, sluggish, foggy, and profoundly tired. When it overperforms (hyperthyroidism), your body runs too hot and burns through energy reserves, which can also leave you exhausted despite feeling wired.
A thyroid-stimulating hormone (TSH) test is the standard screening tool. For healthy, non-pregnant adults, the normal TSH range is roughly 0.27 to 4.2 uIU/mL, though ranges vary slightly between labs. Values above that range suggest an underactive thyroid; values below it suggest an overactive one. Thyroid problems are treatable, and fatigue often improves significantly once levels are corrected.
Depression, Anxiety, and Stress
Mental health conditions don’t just make you feel sad or worried. They physically slow the body down. Depression disrupts the brain chemicals that regulate mood, sleep, and appetite. When those signaling systems malfunction, sleep quality deteriorates even if you’re spending more time in bed, and your body enters a state of low energy that rest alone can’t fix. As Mayo Clinic researchers describe it: “Physically, the body really slows down. You feel tired. Your sleep is often disrupted.”
Anxiety works differently but produces a similar result. Chronic anxiety keeps your stress response activated, flooding your system with stress hormones. That sustained state of alertness is metabolically expensive. Over weeks and months, it depletes your energy reserves. Grief, ongoing emotional stress, and a history of physical or emotional abuse can produce the same pattern of unrelenting fatigue.
Post-Viral Fatigue
If your tiredness started after an illness, especially COVID-19, mononucleosis, or the flu, post-viral fatigue syndrome is a likely explanation. One theory is that the initial infection triggers an immune response that malfunctions and causes long-lasting inflammation even after the virus itself is gone. That lingering inflammation is what keeps you exhausted.
Post-viral fatigue can be diagnosed once symptoms have persisted for at least two weeks, and it can last weeks, months, or in some cases years. Between 10% and 35% of people with post-viral syndrome go on to meet the criteria for chronic fatigue syndrome after six months. Recovery varies widely depending on your overall health, the severity of the original infection, and individual factors that are still poorly understood. Pacing your activity level, rather than pushing through, tends to produce better outcomes.
Chronic Fatigue Syndrome (ME/CFS)
Chronic fatigue syndrome, formally called myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), is a distinct condition, not just “being really tired.” The CDC’s diagnostic criteria require three features: a substantial drop in your ability to function that lasts more than six months, with fatigue that is new (not lifelong) and not relieved by rest; post-exertional malaise, meaning your symptoms crash after physical, mental, or emotional effort, often 12 to 48 hours later and lasting days or weeks; and unrefreshing sleep, where a full night’s rest doesn’t make you feel better.
On top of those three, you need at least one additional symptom: either cognitive impairment (brain fog, memory issues, trouble processing information) or orthostatic intolerance (symptoms worsen when you’re upright and improve when lying down). These symptoms must be present at least half the time at moderate or greater intensity. ME/CFS is a serious, often disabling condition that requires a different management approach than ordinary fatigue.
Other Medical Conditions Worth Checking
Diabetes causes fatigue because your cells can’t efficiently use glucose for energy, whether from insufficient insulin or insulin resistance. Heart disease and heart failure reduce blood flow, meaning your muscles and organs aren’t getting the oxygen they need. Kidney disease allows waste products to build up in the blood, producing a heavy, persistent tiredness that’s hard to shake. Autoimmune conditions like lupus and rheumatoid arthritis drain energy through chronic inflammation.
Medications are another underappreciated cause. Antihistamines, blood pressure drugs, antidepressants, pain medications, and many others list fatigue as a side effect. If your tiredness started around the time you began a new medication, that connection is worth raising with your prescriber. Obesity also contributes independently. Carrying extra weight increases the workload on your heart, lungs, and joints, and it raises the risk of sleep apnea.
What to Do When Fatigue Won’t Lift
Start with the basics: consistent sleep and wake times, regular physical activity (even light walking helps), balanced meals with enough protein and iron-rich foods, and adequate water intake. Cut caffeine after early afternoon. If two weeks of these adjustments don’t make a noticeable difference, it’s time for bloodwork. The most useful initial tests check your blood count (for anemia), thyroid function (TSH), blood sugar, vitamin B12, vitamin D, and ferritin levels. These six tests cover the most common treatable causes of persistent fatigue.
Pay attention to red-flag symptoms that signal something more urgent: shortness of breath, a fast or irregular heartbeat, unexplained weight loss, unusual bleeding, severe headaches, or feeling like you might pass out. These warrant prompt medical evaluation rather than a wait-and-see approach.