Excessive tiredness usually comes from one of a handful of causes: not enough quality sleep, a nutritional deficiency, an underlying medical condition, or a mental health issue like depression. Often, more than one factor is at play. The tricky part is that fatigue is one of the most common symptoms in medicine, shared by dozens of conditions, so pinpointing the cause requires looking at the full picture of your health, habits, and history.
Sleep That Doesn’t Restore You
The most straightforward cause of excessive tiredness is simply not sleeping enough. Adults need at least seven hours per night, and falling short of that consistently counts as insufficient sleep by CDC standards. But plenty of people who spend seven or eight hours in bed still wake up exhausted, and that points to a problem with sleep quality rather than quantity.
Obstructive sleep apnea is one of the most underdiagnosed causes of daytime exhaustion. Your airway partially or fully collapses during sleep, cutting off oxygen repeatedly throughout the night. Each episode triggers a brief arousal from deep sleep, sometimes dozens of times per hour, without you ever fully waking up. The result is that you feel like you slept all night but got almost no restorative rest. Sleep apnea is graded by how many breathing interruptions occur per hour: five to fifteen is mild, fifteen to thirty is moderate, and above thirty is severe. Snoring, gasping during sleep, and waking with a dry mouth or headache are classic signs, but some people have none of these and just feel relentlessly tired.
Iron Deficiency and Anemia
Iron deficiency is one of the most common nutritional causes of fatigue worldwide. Your body needs iron to make hemoglobin, the protein in red blood cells that carries oxygen to your tissues. When iron stores drop, your cells get less oxygen and your energy tanks.
The key blood marker is ferritin, which reflects how much iron your body has in reserve. Levels below 30 ng/mL are a strong indicator of iron deficiency, catching about 92% of cases while remaining highly accurate. In people with chronic inflammation (from autoimmune conditions, for example), iron deficiency is likely when ferritin falls below 50 ng/mL. Levels at or above 100 ng/mL generally rule it out. You can be iron-deficient and fatigued well before you’re technically anemic, which is why checking ferritin specifically matters rather than just looking at hemoglobin.
Thyroid Problems
Your thyroid gland produces hormones that control how your body uses energy, affecting nearly every organ, including your heart. When the thyroid is underactive, a condition called hypothyroidism, many of your body’s functions slow down. Metabolism drops, body temperature regulation falters, and fatigue sets in alongside other symptoms like weight gain, dry skin, constipation, and feeling cold when others are comfortable.
Hypothyroidism develops gradually, so you might not notice a dramatic shift. Instead, the tiredness creeps in over months and becomes your new normal. A simple blood test measuring thyroid-stimulating hormone (TSH) is the first step in checking thyroid function. Because symptoms overlap heavily with depression, sleep disorders, and iron deficiency, thyroid testing is a routine part of any fatigue workup.
Depression and Anxiety
Mental health conditions don’t just make you feel emotionally drained. They cause genuine physical fatigue through measurable biological changes. Depression activates the immune system in ways that mirror a chronic low-grade infection. Inflammatory molecules rise, and they directly interfere with energy regulation in the brain and body.
One well-studied pathway involves tryptophan, an amino acid your body normally uses to make serotonin (which regulates mood) and melatonin (which regulates sleep). In depression, inflammation redirects over 95% of available tryptophan away from serotonin production and down an alternative pathway. The result is a double hit: less serotonin to stabilize mood, and less melatonin to support sleep. This helps explain why depression-related fatigue feels so physical, not like laziness or sadness, but like your body has been unplugged.
Anxiety contributes differently. The constant activation of your stress response burns through energy reserves and keeps your nervous system on high alert, which is exhausting in itself. Many people experience both anxiety and depression simultaneously, compounding the fatigue.
Blood Sugar and Diabetes
High blood sugar disrupts your body’s ability to convert glucose into usable energy. In type 2 diabetes and prediabetes, cells become resistant to insulin, the hormone that lets glucose enter cells. So even though there’s plenty of sugar circulating in your blood, your cells can’t access it efficiently. On top of that, elevated blood sugar triggers increased urination, which leads to dehydration, another contributor to tiredness.
Fatigue from blood sugar issues often follows meals, particularly carbohydrate-heavy ones, and may come with increased thirst, blurred vision, or frequent trips to the bathroom. A fasting glucose test or a hemoglobin A1C test (which shows your average blood sugar over the past two to three months) can identify whether this is a factor.
Vitamin B12 Deficiency
Vitamin B12 is essential for maintaining the protective coating around your nerves (called myelin) and plays a role in producing red blood cells. When levels drop, you can develop a type of anemia similar to iron deficiency, along with neurological symptoms like tingling in your hands and feet, difficulty concentrating, and persistent fatigue.
B12 comes almost exclusively from animal-based foods, so vegans and vegetarians are at higher risk. But absorption problems are just as common a cause, especially in older adults or people taking certain medications like proton pump inhibitors (heartburn drugs) or metformin for diabetes. B12 deficiency can develop slowly over years because your liver stores enough to last a long time, meaning symptoms often appear long after the deficiency began.
Post-Viral Fatigue and Long COVID
Some infections leave behind a fatigue that outlasts the illness by months or even years. Long COVID is the most widely recognized example right now, defined as symptoms lasting at least three months after a SARS-CoV-2 infection. But this pattern isn’t unique to COVID. Epstein-Barr virus (which causes mono), influenza, and other infections can trigger a similar prolonged exhaustion.
The hallmark of post-viral fatigue is something called post-exertional malaise: a crash in energy that follows physical or mental activity, often hitting one to two days after the effort. This distinguishes it from ordinary tiredness, where rest helps. With post-viral fatigue, pushing through often makes things worse. There’s no lab test that confirms Long COVID. Diagnosis is based on your history, symptoms, and ruling out other conditions. Some people with Long COVID meet the criteria for myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), a condition that has been recognized for decades but remains poorly understood.
Medications That Drain Your Energy
Several commonly prescribed medications list fatigue or drowsiness as a side effect. Among the most notable:
- Antihistamines like diphenhydramine (found in many over-the-counter sleep aids and allergy medications) cause significant next-day drowsiness, even impairing driving performance the following morning.
- Beta-blockers used for high blood pressure and heart conditions commonly cause tiredness and daytime sleepiness.
- Certain antidepressants, including some SSRIs, can be sedating enough to affect alertness and driving ability.
If your tiredness started or worsened around the time you began a new medication, that connection is worth exploring. Sometimes adjusting the dose or timing, or switching to an alternative, resolves the fatigue entirely.
How Doctors Investigate Fatigue
When you see a doctor about persistent tiredness, expect a conversation about your sleep habits, mood, stress levels, diet, and medication list before any blood is drawn. That history often narrows things down considerably.
The standard blood workup for fatigue typically includes a complete blood count (to check for anemia), a comprehensive metabolic panel (which measures 14 substances including glucose, kidney function markers, liver enzymes, and electrolytes), thyroid function, iron studies with ferritin, and vitamin B12. If diabetes is suspected, a hemoglobin A1C test is added. These tests together can identify or rule out the most common medical causes in a single visit.
If everything comes back normal, the conversation often shifts to sleep quality, mental health screening, and lifestyle factors like physical activity and alcohol use. A sleep study may be recommended if sleep apnea is suspected. In some cases, no single clear cause emerges, and the fatigue turns out to be driven by a combination of borderline deficiencies, poor sleep habits, and chronic stress acting together.