Understanding Fatigue: More than just feeling tired

“I sleep eight hours, but I still wake up exhausted.”

This is a complaint doctors hear often. Fatigue is more than just being tired—it’s a persistent feeling of physical or mental exhaustion that interferes with daily life.

While many people attribute fatigue to being overworked or not sleeping well, the reality is that fatigue can stem from a wide variety of causes—ranging from nutritional deficiencies and hormonal imbalances to mental health conditions, sleep disorders, and even heart disease.

Understanding the root cause is essential because when addressed properly, patients often experience dramatic improvements in energy and overall quality of life.

Common Causes of Fatigue

Nutritional & Blood-Related Causes

Iron Deficiency Anemia
Iron deficiency is one of the most common causes of fatigue, often due to blood loss. Examples include heavy menstrual bleeding in women, gastrointestinal bleeding from ulcers or hemorrhoids, inflammatory bowel disease, or even an undetected malignancy. Diagnosis usually involves checking hemoglobin and iron studies, and depending on the patient’s history, further imaging or endoscopy may be recommended. Treatment focuses on replenishing iron stores, either through iron-rich foods like red meat, beans, lentils, fortified cereals, leafy greens, and dried fruits, or with iron supplementation when diet alone is insufficient.
Vitamin B12 Deficiency
Vitamin B12 is essential for making healthy red blood cells. Deficiency may cause fatigue, anemia, and neurological symptoms. Because B12 is primarily found in animal-based foods, strict vegetarians are at higher risk. Diagnosis is made with a blood test, and treatment often begins with vitamin B12 injections, later transitioning to oral supplements once levels are stabilized.

Vitamin D Deficiency
Low vitamin D levels are common in regions with long winters or limited sun exposure. Deficiency can contribute to fatigue, muscle aches, and low mood. A blood test can confirm low vitamin D levels, and supplementation usually improves both energy and overall well-being.

Endocrine causes

Hypothyroidism

Hypothyroidism, a condition in which the thyroid gland does not produce enough hormone, slows down metabolism and leads to persistent tiredness, weight gain, and other symptoms. It is easily diagnosed with a blood test measuring thyroid-stimulating hormone (TSH). Treatment involves thyroid hormone replacement therapy, which often results in significant improvement in energy levels once hormone balance is restored.

Psychological causes

Depression and Anxiety

Depression and anxiety are common but often overlooked causes of fatigue. Patients rarely say, “I feel depressed.” Instead, they may describe fatigue, low energy, or a loss of interest in activities they once enjoyed. Careful screening for mental health conditions is essential whenever fatigue is a primary concern. Management may involve counseling, mindfulness, stress-reduction strategies, and, in some cases, antidepressant or anti-anxiety medication prescribed by a healthcare provider.

Sleep-Related Causes

Insomnia and poor sleep hygiene

These are frequent culprits in patients who get only four or five hours of sleep per night and then wonder why they feel drained. Improving sleep habits can make a big difference. This may include setting a consistent bedtime, limiting caffeine in the afternoon, reducing screen time before bed, and creating a restful environment. For some individuals, short-term use of sleep aids may also be considered.

Sleep apnea

Obstructive sleep apnea is another underdiagnosed but significant cause of fatigue. In this condition, breathing repeatedly stops during sleep, reducing oxygen levels and disrupting rest. Often, it is a partner who notices loud snoring or pauses in breathing. A sleep study is used for diagnosis, and treatment typically involves Continuous Positive Airway Pressure (CPAP) therapy. Many patients report dramatic improvement in both fatigue and overall health once therapy begins.

Cardiac Causes

Congestive Heart Failure & Atrial Fibrillation

In heart failure, the weakened heart cannot pump enough blood to supply the body’s tissues, causing fatigue, shortness of breath, and exercise intolerance.

In atrial fibrillation, the irregular heartbeat reduces circulation and oxygen delivery, leading to persistent tiredness.

When to See a Doctor for Fatigue

While occasional tiredness is normal, you should see a doctor if fatigue is persistent, worsening, or affecting daily life. Seek medical attention promptly if fatigue is accompanied by:

Unexplained weight loss
Night sweats or persistent fever
Chest pain, palpitations, or shortness of breath
Severe snoring or pauses in breathing during sleep
Weakness, numbness, or neurological changes
Pale skin or signs of anemia
Low mood, hopelessness, or loss of interest in daily activities

These may indicate a serious medical or mental health condition requiring timely evaluation.

Sources:

Latimer, K.M., Gunther, A. and Kopec, M. (2023) Fatigue in adults: Evaluation and management, American Family Physician. Available at: https://www.aafp.org/pubs/afp/issues/2023/0700/fatigue-adults.html (Accessed: 03 September 2025).

Maisel P, Baum E, Donner-Banzhoff N. Fatigue as the Chief Complaint–Epidemiology, Causes, Diagnosis, and Treatment. Dtsch Arztebl Int. 2021 Aug 23;118(33-34):566-576. doi: 10.3238/arztebl.m2021.0192. PMID: 34196270; PMCID: PMC8579431.