What causes a person to feel lethargic? A comprehensive guide

September 23, 2025 •

4 min read

Millions of people worldwide experience lethargy, an overwhelming state of low energy and drowsiness that does not simply go away with rest. Uncovering what causes a person to feel lethargic is the crucial first step toward recovery and restoring your vitality.

Quick Summary

Lethargy can stem from numerous factors, including poor sleep habits, chronic stress, nutritional deficiencies, and underlying medical conditions like anemia or thyroid disorders. Pinpointing the root cause is key to effective management and restoring your energy levels.

Key Points

Differentiate Lethargy: Understand that lethargy involves a decrease in mental alertness and is more than just feeling tired, often pointing to a deeper health issue.
Assess Your Lifestyle: Common culprits include poor sleep hygiene, dehydration, an unhealthy diet, excessive stress, and an imbalanced exercise routine.
Check for Medical Causes: Lethargy can be a sign of underlying conditions like anemia, thyroid disorders, diabetes, infections, and heart or lung disease.
Mind Your Mental Health: Psychological issues such as depression, anxiety, and grief can significantly impact energy levels and must be addressed.
Identify Nutritional Gaps: Deficiencies in vital nutrients like iron, vitamin B12, vitamin D, and magnesium are frequent causes of low energy.
Prioritize Quality Sleep: Addressing sleep disorders like sleep apnea or insomnia is critical, as quality rest is key to combating persistent lethargy.
Know When to Seek Help: Consult a doctor if lethargy is persistent, sudden, or accompanied by other severe symptoms, as some causes are time-sensitive.

Understanding the difference between lethargy and tiredness

Before diving into the causes, it's important to differentiate between lethargy, fatigue, and normal tiredness. Tiredness is a temporary state that is typically resolved with rest or sleep. Fatigue is a persistent, unexplained exhaustion lasting for months, which is not substantially relieved by rest. Lethargy, however, is a more complex symptom involving not only a lack of energy but also decreased mental alertness, drowsiness, and a potential reduction in overall consciousness. It often indicates a deeper issue affecting the brain or other bodily functions.

Lifestyle and environmental factors

Many people can trace their low energy levels back to their daily habits and external environment. These factors are often controllable and can be addressed through lifestyle adjustments.

Sleep habits

Poor sleep is a primary driver of lethargy. This can be caused by a variety of issues, such as:

Insufficient sleep: Most adults require 7 to 9 hours of sleep per night. A chronic lack of sleep prevents the body from fully recovering and leads to daytime drowsiness.
Sleep disorders: Conditions like insomnia, obstructive sleep apnea (which causes repeated breathing interruptions during sleep), and restless legs syndrome can disrupt sleep quality and leave you feeling unrefreshed.
Poor sleep hygiene: Irregular sleep schedules, caffeine consumption late in the day, or using electronic devices before bed can all negatively impact sleep architecture.

Diet and nutrition

The fuel we provide our bodies directly influences our energy production. Nutritional deficiencies and poor dietary choices can cause lethargy.

Nutrient deficiencies: A lack of essential vitamins and minerals, especially iron (leading to anemia), vitamin B12, vitamin D, and magnesium, can significantly decrease energy levels.
Dehydration: Not drinking enough water is a common and often overlooked cause of fatigue and low energy.
Poorly balanced diet: Diets high in refined carbohydrates and sugar can cause a quick energy spike followed by a crash, leading to lethargy. A balanced diet of whole grains, protein, and healthy fats helps maintain stable energy.

Physical activity

Both too much and too little exercise can contribute to feeling lethargic. A sedentary lifestyle reduces the body's overall energy and stamina, while overexertion can lead to muscle fatigue and burnout. Moderate, consistent physical activity is key to boosting energy.

Chronic stress

Stress is the body's natural response to perceived threats, but chronic stress can lead to severe energy depletion. The constant fight-or-flight mode elevates stress hormones like cortisol, which can disrupt sleep, increase irritability, and cause severe exhaustion over time.

Underlying medical conditions

If lifestyle changes don't resolve lethargy, an underlying medical condition may be the culprit. A doctor's evaluation is necessary to diagnose and treat these issues.

Endocrine disorders

Thyroid problems: An underactive thyroid gland (hypothyroidism) slows down your metabolism, leading to fatigue, weight gain, and an inability to tolerate cold.
Diabetes: Fluctuations in blood sugar levels can cause significant tiredness and lethargy, especially in uncontrolled diabetes.

Anemia and other blood conditions

Anemia, often caused by an iron deficiency, means your blood lacks enough healthy red blood cells to carry adequate oxygen to your body's tissues, resulting in weakness and exhaustion.

Heart and lung diseases

Conditions that affect the heart's ability to pump blood or the lungs' ability to deliver oxygen, such as heart disease and COPD, can manifest as profound fatigue.

Infections and autoimmune diseases

Chronic infections (e.g., Lyme disease, mononucleosis, post-viral fatigue from illnesses like COVID-19) and autoimmune diseases (e.g., lupus, rheumatoid arthritis) can cause persistent fatigue and lethargy as the body's immune system is constantly engaged.

Medications

Certain prescription drugs, including some antidepressants, antihistamines, and beta-blockers, can cause drowsiness and lethargy as a side effect.

Psychological and mental health triggers

Mental well-being is intrinsically linked to physical energy. Psychological conditions often manifest with physical symptoms like lethargy.

Depression and anxiety: Both conditions can drain mental and physical energy, leading to a profound lack of motivation and persistent tiredness.
Grief: The emotional and mental strain of grieving a loss can cause significant exhaustion.

A comparison of lethargy, fatigue, and tiredness


Feature	Tiredness	Fatigue	Lethargy
Duration	Short-term, temporary	Persistent, long-term (weeks or months)	Can be acute or chronic
Cause	Normal physical or mental exertion, lack of sleep	Often points to an underlying medical or psychological issue	Disruption of brain function; can be a sign of serious illness
Sensation	Feeling sleepy or worn out	Constant, unexplained exhaustion, not relieved by rest	Low energy, drowsiness, and decreased mental alertness
Effect	Responds to a good night's sleep	Impairs daily activities, sleep does not fully resolve it	Difficulty concentrating, mental fog, and apathy

When to see a doctor

While lifestyle changes can often improve low energy, it's crucial to consult a healthcare provider if your lethargy is persistent, worsens, or is accompanied by other symptoms. Immediate medical attention is necessary if lethargy is sudden and accompanied by:

Changes in mental status (confusion, unresponsiveness)
Chest pain or shortness of breath
Rapid heart rate
Severe pain or high fever
Slurred speech or muscle weakness

Your doctor can perform tests to rule out serious conditions and develop a treatment plan. For more information on differentiating between fatigue types, you can consult authoritative health resources, such as the Cleveland Clinic.

Conclusion: Regaining your vitality

Lethargy is not an inevitability, but a symptom that requires careful attention. By evaluating your lifestyle habits, diet, and mental health, and by consulting a healthcare professional when needed, you can identify the root cause. This knowledge empowers you to take proactive steps—whether through simple adjustments or medical treatment—to restore your energy and reclaim your daily life.

Frequently Asked Questions

While temporary low energy can be normal, you should see a doctor if your lethargy persists for more than a couple of weeks, worsens, or is accompanied by other severe symptoms like chest pain, confusion, or a high fever.

Yes, deficiencies in certain key nutrients like iron, vitamin B12, vitamin D, and magnesium can significantly affect your energy metabolism and be the primary cause of lethargy.

Yes, lethargy is a very common symptom of depression and anxiety. These mental health conditions can drain both physical and mental energy, leading to a persistent feeling of exhaustion.

Sleep apnea and other sleep disorders disrupt the body's natural sleep cycle. Even if you are in bed for enough hours, the poor quality of sleep prevents your body from properly resting and recovering, leading to daytime lethargy.

Yes. Dehydration is a common and often overlooked cause of low energy. The body needs sufficient fluids to function optimally, and even mild dehydration can result in fatigue and lethargy.

Lethargy is a medical symptom characterized by a profound lack of energy and mental alertness that is beyond your control. Laziness is typically a choice or lack of motivation despite having sufficient energy.

Yes, an underactive thyroid gland (hypothyroidism) is a very common cause of lethargy. This condition slows down your metabolism and other bodily functions, leading to feelings of fatigue and sluggishness.