11 Causes of Sudden Fatigue or Exhaustion

As a symptom, fatigue is a tough one. “Fatigue is defined as tiredness, exhaustion, or lack of energy precipitated by exertion or stress,” says Andrew Patane, MD, an internal medicine doctor at NYU Langone Health in New York City. “It's a subjective symptom and, unfortunately, we don't have objective [ways to measure it with an exam].”

Lifestyle factors, medications, and health conditions can all cause extreme fatigue. Here are 11 potential underlying causes.

1. Illness or Infection

Sudden, overwhelming fatigue could be a sign of a viral or bacterial infection.

According to Cleveland Clinic, some examples of viral infections that often cause tiredness are:

• Flu
• COVID-19
• Lyme disease
• Mononucleosis (mono)

Fever is common with all of these illnesses, but other signs and symptoms may vary.

Sudden, severe fatigue can also happen along with serious bacterial infections, according to the Sepsis Alliance. Examples of these types of infections are:

• Urinary tract infections (including bladder and kidney infections)
• Bacterial pneumonia
• Dental abscesses
• Sinus infections

In addition to fever, most bacterial infections cause chills and pain or discomfort in the affected area.

2. A Vitamin or Nutrient Deficiency

A vitamin deficiency, such as not getting enough vitamin D or vitamin B12, could cause you to feel severely tired, according to Cleveland Clinic. Dehydration can have the same effect.

You could also feel suddenly exhausted if you've recently started a high-protein diet. It's not the protein itself making you tired, but the lack of carbohydrates. Severely restricting carbs causes your body to enter ketosis, where it breaks down fat for energy, according to Mayo Clinic. The side effects of ketosis (sometimes called “keto flu”) include fatigue, dizziness, weakness, and “brain fog,” among others, according to one study.

These side effects are usually temporary. But if ketosis is causing your sudden energy crash, you may want to add more fiber-rich, complex carbohydrates to your diet. Those include fruits, vegetables, and whole grains like oatmeal and brown rice. The Dietary Guidelines for Americans recommend that carbs make up 45 to 65 percent of your total daily calories.

Similarly, skipping meals or following a very low-calorie diet could sap your energy. Women should try to avoid taking in fewer than 1,200 calories per day, and men should try to avoid taking in fewer 1,500 calories per day, according to Harvard Health Publishing.

But keep in mind that those are bare minimums. The Dietary Guidelines for Americans recommend that adult women eat somewhere between 1,600 and 2,400 calories a day, and men get 2,000 to 3,000 calories daily, depending on age and activity level. Check the guidelines' chart of estimated calorie needs, or use a calorie counting app.

3. Certain Lifestyle Habits

This is one of the first areas Dr. Patane says he checks on when he sees patients for fatigue. He may ask them if they are:

• Drinking more alcohol than usual
• Doing extreme workouts, such as training for an endurance race
• Under-eating
• Working more than usual
• Doing night-shift work
• Having a work-life imbalance
• Practicing poor sleep habits

While some of these might sound obvious, it sometimes takes the help of a professional to point these out and guide you in making the right changes.

4. A New Medication

Medications are meant to make you feel better, but starting a new prescription drug often causes fatigue as a side effect.

Some medications that can cause fatigue, according to Patane, include:

• Antidepressants
• Antihistamines
• Beta-blockers for high blood pressure
• Benzodiazepines for anxiety

If you've recently started a new medication, talk to your doctor. They may be able to change your prescription or alter the dose to treat you while minimizing these side effects.

5. Stress, Anxiety, or Depression

Mental health issues are known for dragging down your energy. “These can all present with fatigue,” Patane says.

Stress is also a key component of fatigue. A build-up of stress over time without a proper release or self-care measures can leave you feeling emotionally exhausted and severely tired, according to Mayo Clinic Health System.

6. Heart Attack or Stroke

While you may be on the lookout for chest pain to signal problems with your heart, you should also add sudden-onset fatigue to the list, according to the American Heart Association (AHA). (Other symptoms of a heart attack can include dizziness, lightheadedness, and pressure in your upper back).

Fatigue or confusion can also be one of the initial symptoms of a stroke, according to the AHA. Other symptoms might include a severe headache, arm weakness, and trouble with walking or talking.

If you have sudden exhaustion along with any other symptoms of a heart attack or stroke, call 911 immediately. In either case, your likelihood of recovery is better the sooner you get treatment.

7. Autoimmune Diseases

More than 100 conditions are considered autoimmune diseases — multiple sclerosis, ulcerative colitis, type 1 diabetes, and lupus are just a few, according to the Autoimmune Association. In all cases, they're caused when your immune system attacks your body.

Fatigue is the most-commonly reported symptom across these diseases, and sometimes the sudden bouts of extreme fatigue can be profound and debilitating. The reason? High levels of inflammation contribute to the kind of tiredness that can make carrying out everyday tasks hard.

These conditions can be tricky to receive a diagnosis for, in part because symptoms, like fatigue, can be so non-specific. They do tend to run in families, though, so be sure to tell your doctor about your family health history.

8. Anemia

Iron-deficiency anemia is defined as not having enough iron in your body, according to the National Heart, Lung, and Blood Institute. And fatigue is the most-frequent symptom.

The condition can cause sudden lethargy and is more common in menstruating women, endurance athletes, and people with kidney or digestive diseases.

Your doctor can diagnose you based on a physical exam and blood tests. They may recommend iron supplements or diet changes to get your iron stores and your energy back up.

9. Postural Orthostatic Tachycardia Syndrome (POTS)

POTS is a group of symptoms that occur when your blood doesn't circulate properly, according to Johns Hopkins Medicine. Doctors don't fully understand the condition or why it happens. But they know that when a person with POTS stands upright, more blood collects in their lower body, causing their heart to beat faster to pump blood up to the brain.

Some people with POTS describe it as having sudden fatigue attacks. Others say it's like having waves of sudden extreme fatigue and nausea, or extreme fatigue “out of nowhere.” It can present differently depending on the person, but other symptoms might include:

• A feeling of lightheadedness or dizziness when standing, or fainting
• Nausea and vomiting
• Brain fog
• Muscle cramps or muscle pain
• Headaches
• A shaky feeling
• Sweating a lot

POTS tends to run in families, and there's a link between POTS and having highly mobile joints (Ehlers-Danlos syndrome), according to Johns Hopkins Medicine. If you think you might have it, see your doctor, who can run a test and make a diagnosis.

10. Chronic Fatigue Syndrome

The hallmark symptom of chronic fatigue syndrome (CFS), also known as myalgic encephalomyelitis (ME), is profound fatigue that lasts at least six months, doesn't go away with rest, and can't be explained by an underlying medical condition. While it's a long-term condition, it may come on suddenly.

The exact causes of CFS aren't known, but in some cases it's thought to develop after a viral or bacterial infection, or due to genetics or a metabolic problem, according to Mayo Clinic.

Other symptoms of CFS include:

• Problems with memory or thinking skills
• Dizziness
• Muscle or joint pain
• Trouble sleeping or sleep that isn't refreshing

11. Cancer

When there's something wrong, it can be hard not to think about the worst-case scenario. So it's important to get checked out by your doctor before making any assumptions. But when you're telling your doctor about your symptoms, they may wonder if there’s any reason to suspect cancer.

Some red flags — which do not point to a diagnosis, only to the possibility of more testing — include fatigue that's accompanied by fevers or night sweats, unintended weight loss, or lumps and bumps under your armpits and the side of your neck, according to the American Cancer Society. If you have any of these, see your doctor sooner rather than later.

When to Call Your Doctor

If fatigue keeps you from taking part in work or school, or social or personal activities, then you should see your doctor, Patane says.

“When in doubt, it never hurts to speak to your doctor about this. It can be a simple evaluation, and we may be able to say there's nothing seriously wrong and suggest lifestyle modifications. One of the best ways to [address] fatigue is with good sleep hygiene and by slowly increasing daily exercise.”