fatigue tired resting
If you're feeling fatigue, you should first try getting more sleep and exercise.
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  • Fatigue is often caused by lifestyle factors like a lack of exercise, not getting enough sleep, being overweight, or feeling stressed. 
  • Fatigue is also a common symptom of many medical conditions, including respiratory illnesses like the flu, mental health conditions like depression, and chronic diseases like cancer. 
  • If you’re experiencing fatigue along with a fever, shortness of breath, headaches, or other concerning symptoms, you should check in with a doctor. 
  • Visit Insider’s Health Reference library for more advice

It’s natural to feel very tired or weak from time to time, but fatigue does not describe the same feeling. 

“The terms fatigue and tired are often used interchangeably, but have different meanings medically,” says Lawrence Epstein, MD, medical director of clinical sleep medicine at Brigham and Women’s Hospital and past president of the American Academy of Sleep Medicine.

While feeling tired is a temporary feeling that can usually be relieved by getting a good night’s sleep, fatigue is an ongoing condition. It makes you experience an overwhelming lack of energy and motivation. You may be unable to concentrate, feel anxious, and have difficulty sleeping.

Here’s what you need to know about the major causes of fatigue. 

What causes fatigue? 

There are many possible causes of fatigue, from common lifestyle factors to underlying medical conditions. 

"It can be a normal response to physical activity, but also can be related to emotional stress, certain medical conditions, or lack of sleep," Epstein says. 

Although the physiological and psychological mechanisms that lead to fatigue are not fully understood, researchers have found that inflammation has an important role, which may explain why fatigue is a common symptom for many illnesses. 

Lifestyle factors

If you think you may have fatigue, Epstein says you should first make sure you are getting enough sleep and regular exercise, and see if your energy levels improve. Lack of sleep and a sedentary lifestyle are two of the most common non-medical causes of fatigue.


For reference, the National Sleep Foundation recommends that adults get seven to nine hours of sleep every night, and the American Heart Association recommends that adults engage in physical activity for at least 2.5 hours each week. If you're consistently meeting these recommendations yet still feel tired, it could be fatigue. 

Consuming foods and drinks that contain ingredients like refined sugar and caffeine can also make you feel fatigued. To help boost your energy, try cutting back on these and eating more foods with whole grains, healthy unsaturated fats, and other beneficial ingredients to boost your energy. Eating small meals throughout the day may also help.

These are some of the other lifestyle factors that can cause fatigue:

  • Being overweight. "Increasing weight requires higher energy expenditures for any activity, increasing likelihood of fatigue," Epstein says.
  • Jet lag
  • Stress
  • Medications like antihistamines and cough medicine
  • Regular use of alcohol and drugs, such as cocaine

If you're still feeling tired after making changes to your lifestyle, you should see your doctor for an evaluation, as it may indicate an underlying medical condition. 

Medical conditions 

If a medical condition is causing your fatigue, you will typically have other symptoms, depending on the condition. 

For example, if your fatigue is caused by arthritis, your joints will be stiff and painful. If it's caused by diabetes, you may have symptoms such as frequent urination or feeling very thirsty or hungry. 

The following are some of the medical conditions that can cause fatigue: 

  • Anemia
  • Arthritis
  • Autoimmune diseases such as hypothyroidism, lupus, and rheumatoid arthritis 
  • Anorexia or other eating disorders
  • Cancer. Between 80% and 100% of people with cancer experience fatigue, according to the American Cancer Society.
  • Chronic fatigue syndrome (CFS). A disorder in which fatigue symptoms last for at least six months, don't improve with rest or other lifestyle changes, and any medical causes are ruled out. 
  • Respiratory illnesses like the flu or COVID-19 
  • Depression
  • Diabetes
  • Fibromyalgia
  • Kidney disease
  • Liver disease

When to see a doctor 

The Mayo Clinic recommends seeing your doctor if you have any of the following symptoms along with fatigue, which could indicate that you may have one of the above medical conditions:

  • A fever
  • Unintentional weight loss
  • Difficulty sleeping through the night
  • Constant headache
  • Feeling depressed

You should go to the ER right away if you have any of the following symptoms along with fatigue, Epstein says. These symptoms could indicate that you may be having a heart attack, a blood clot in your lung (pulmonary embolism), or another serious health issue:

  • Chest pain
  • Shortness of breath
  • Change in mental status

Insider's takeaway

The most important thing to know about fatigue, Epstein says, is that it can be related to either lifestyle issues or chronic health conditions. 

If making changes to your lifestyle doesn't help relieve your fatigue, or you think your fatigue may be due to a medical condition, you should see a doctor to help determine what's causing it, Epstein says.

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