What Causes Fatigue? Conditions & Triggers

Fatigue is one of the most common reasons people seek medical help — yet it remains one of the most misunderstood symptoms in healthcare. It is not simply feeling tired after a long day. True fatigue is a persistent, often overwhelming lack of energy that interferes with daily life, work, and relationships. Understanding the causes of fatigue is the first step toward doing something about it.

Fatigue can be temporary or chronic (lasting six months or more). In many cases, small changes to diet, sleep, or medication resolve it quickly. In others, an underlying medical condition requires diagnosis and treatment.

Lifestyle Habits That Drain Your Energy

Many people experiencing fatigue don't realise their daily habits are the root cause. Lifestyle factors are among the most common and correctable contributors to low energy. The good news is that these causes of fatigue are often the easiest to address.

Common lifestyle-related triggers include:

• Poor sleep hygiene — irregular sleep schedules, excessive screen time before bed, or sleeping in environments that are too warm or bright
• Physical inactivity — a sedentary lifestyle paradoxically depletes energy over time
• Poor nutrition — diets high in processed foods and low in essential nutrients leave the body running on empty
• Excessive alcohol consumption — alcohol disrupts sleep architecture and depletes B vitamins
• Chronic stress — prolonged psychological stress keeps the body in a low-grade fight-or-flight state, burning energy reserves
• Dehydration — the body requires adequate fluid to support every metabolic process; even mild dehydration causes notable fatigue

Identifying and modifying these habits can produce a dramatic improvement in energy levels — sometimes within days. If fatigue persists after lifestyle changes, a deeper investigation is warranted.

Sleep Disorders: When Rest Doesn't Restore

Sleep disorders are a major and frequently overlooked cause of chronic fatigue. Unlike ordinary tiredness, fatigue from sleep disorders does not improve with a single good night of sleep. The exhaustion accumulates over weeks and months.

Key sleep disorders linked to extreme fatigue include:

• Insomnia — difficulty falling or staying asleep, leaving the body without adequate restorative rest
• Obstructive sleep apnoea — repeated interruptions in breathing fragment sleep, preventing deep restorative stages
• Restless legs syndrome — uncomfortable sensations in the limbs that make it nearly impossible to fall or stay asleep
• Narcolepsy — a neurological disorder causing sudden, uncontrollable episodes of sleepiness during the day
• Circadian rhythm disorders — misalignment between internal body clocks and external schedules (common in shift workers)

If you wake unrefreshed regardless of how long you sleep, a sleep study (polysomnography) may reveal an underlying disorder. Treating the sleep condition often resolves the fatigue entirely.

Medications and Treatments That Cause Fatigue

Prescription medications are a surprisingly common — and often missed — cause of fatigue. Patients and clinicians alike may attribute tiredness to an illness rather than the drug used to treat it. Always review your medication list when investigating the causes of fatigue.

Prescription medications known to cause fatigue include:

• Benzodiazepines (e.g., diazepam, lorazepam)
• Sedative-hypnotics (e.g., zolpidem)
• Antipsychotics (e.g., quetiapine, olanzapine)
• Anxiolytics
• Opioid painkillers
• Anticonvulsants (e.g., gabapentin, pregabalin)
• Beta-blockers (e.g., metoprolol, atenolol)

Over-the-counter medications can also cause significant drowsiness. Antihistamines — widely used for allergies and colds — are a classic example. Medical treatments such as chemotherapy and bone marrow transplants are associated with profound, long-lasting fatigue that can persist well into recovery.

If you suspect a medication is causing your fatigue, speak with your prescriber before stopping any treatment.

Medical Conditions That Cause Fatigue

Hundreds of medical conditions list fatigue as a core symptom. Below are the major categories and the specific conditions most commonly responsible.

Infections

Fatigue is a hallmark immune response to infection. The body diverts energy toward fighting pathogens, leaving less available for normal daily function. Common infectious causes include influenza, COVID-19, Epstein-Barr virus (mononucleosis), HIV, Lyme disease, and hepatitis.

Post-viral fatigue is increasingly recognised as a distinct syndrome — most visibly in the form of Long COVID, where fatigue persists for months after the acute infection has resolved.

Heart and Lung Conditions

When the heart or lungs fail to deliver sufficient oxygen to tissues, fatigue is inevitable. Cardiovascular and respiratory causes include heart failure, coronary artery disease, chronic obstructive pulmonary disease (COPD), pulmonary hypertension, and arrhythmias. Fatigue in these cases is often accompanied by breathlessness, chest tightness, or reduced exercise tolerance.

Mental Health Conditions

The link between mental health and physical energy is direct and well-documented. Depression is one of the leading causes of fatigue worldwide; it disrupts sleep, appetite, and motivation simultaneously. Anxiety disorders keep the nervous system in a constant state of arousal, which exhausts the body over time. Burnout, post-traumatic stress disorder (PTSD), and grief are also strongly associated with debilitating tiredness.

Fatigue from mental health conditions can make even simple daily tasks feel impossible. Treatment — including therapy, medication, or both — typically improves energy alongside mood.

Autoimmune Disorders

In autoimmune disease, the immune system attacks the body's own tissues — a process that consumes enormous energy and causes chronic inflammation. Fatigue is one of the most disabling symptoms reported by people with:

• Rheumatoid arthritis
• Lupus (SLE)
• Multiple sclerosis
• Inflammatory bowel disease (Crohn's disease and ulcerative colitis)
• Sjögren's syndrome
• Coeliac disease

Managing the underlying inflammatory process is central to managing autoimmune fatigue.

Hormonal Imbalances

The endocrine system governs energy metabolism at a fundamental level. When hormone-producing glands malfunction, fatigue is almost always a consequence. The most common hormonal cause of fatigue is hypothyroidism — an underactive thyroid gland that slows virtually every metabolic process in the body.

Other hormonal causes include:

• Type 1 and type 2 diabetes
• Adrenal insufficiency (Addison's disease)
• Cushing's syndrome
• Polycystic ovary syndrome (PCOS)
• Menopause-related hormonal shifts

Blood tests can identify most of these conditions, and treatment — typically hormone replacement or regulation — produces marked improvements in energy.

Other Chronic Conditions

Several chronic conditions cause severe, long-lasting fatigue that significantly impairs quality of life. These include:

• Fibromyalgia — widespread musculoskeletal pain with persistent fatigue
• Myalgic encephalomyelitis / chronic fatigue syndrome (ME/CFS) — a complex neurological condition defined largely by its disabling fatigue
• Chronic kidney disease
• Liver disease
• Cancer (and cancer-related treatments)

ME/CFS deserves special mention because fatigue is the defining feature rather than a secondary symptom. People with ME/CFS often experience post-exertional malaise — a worsening of symptoms after even minor physical or mental effort.

Nutritional Deficiencies, Weight, and Eating Disorders

What you eat — and how much — has a profound effect on your energy levels. Nutritional causes of fatigue are extremely common and frequently go undiagnosed without targeted blood testing.

Key deficiencies linked to fatigue include:

• Iron-deficiency anaemia — insufficient iron limits the blood's ability to carry oxygen to tissues
• Vitamin B12 deficiency — critical for nerve function and red blood cell production; deficiency causes profound fatigue and cognitive fog
• Vitamin D deficiency — increasingly common in populations with limited sun exposure; strongly associated with tiredness and low mood
• Folate deficiency — impairs cell production and energy metabolism
• Magnesium deficiency — magnesium is involved in over 300 enzymatic reactions; low levels cause muscle weakness and fatigue

Eating disorders create fatigue through multiple overlapping mechanisms. Anorexia and bulimia deprive the body of essential calories and micronutrients. Being significantly underweight or overweight also places additional strain on physiological systems, contributing to persistent tiredness.

When to See a Healthcare Provider

Fatigue that persists beyond two weeks without an obvious cause warrants medical evaluation. Prompt investigation is especially important when fatigue is accompanied by unexplained weight loss, fever, night sweats, chest pain, breathlessness, or significant changes in mood or cognition.

A healthcare provider will typically begin with a thorough history and physical examination, followed by targeted blood tests — checking for anaemia, thyroid function, blood glucose, inflammatory markers, and vitamin levels. From there, further specialist investigations may follow.

The vast majority of fatigue causes are treatable — or at minimum, manageable. Receiving a diagnosis is the most important step toward reclaiming your energy.

The Bottom Line

The causes of fatigue span an enormous range — from correctable lifestyle habits to serious chronic disease. No single factor explains fatigue in every person, which is why a systematic approach to investigation matters. Start with the basics: sleep, hydration, nutrition, and medication review. If those don't explain it, pursue medical evaluation without delay. Fatigue is a signal, not a character flaw — and in most cases, it is a signal the body is sending for a very good reason.