Lethargy is a general state of fatigue that involves a lack of energy and motivation for physical and mental tasks. People may also describe this as “feeling lethargic”.

Some simple causes, such as overexertion or not getting enough sleep, can leave a person feeling lethargic. In other cases, an underlying health condition may cause long-term lethargy and fatigue.

It may be necessary to see a doctor to determine the cause of lethargy if it lasts for a longer period of time. Getting a full diagnosis can help identify any underlying issues and work toward treatment.

Keep reading to learn more about the possible causes of lethargy, how to treat them, and when to see a doctor.

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Lethargy refers to a state of fatigue and low energy that can prevent a person from doing daily tasks.

Lethargy refers to a sense of fatigue and a lack of both mental and physical energy and motivation. A person with lethargy may feel unmotivated or have an indifferent attitude toward their daily activities.

They may feel as though they are walking around in a haze or constant state of brain fog and do not have the energy to complete their normal tasks.

There are a number of potential causes of lethargy, ranging from a temporary state of fatigue to a chronic health condition.

Lethargy is a subjective symptom, meaning that individual people will measure and describe it differently. Because of this, there is no concrete way to test for the symptom.

The sections below list some factors and conditions that may lead to a person feeling lethargic.


Overexertion may be a simple cause of lethargy. A person who overexerts themselves physically may feel as though they have drained their energy stores, leading to a low, sluggish state as they recover.

Some causes of overexertion may include:

  • completing very strenuous workouts
  • walking or hiking great distances
  • spending lots of time in the sun
  • working long hours
  • doing repetitive tasks

Many other common issues may lead to short-term overexertion and lethargy.

This is a relatively straightforward cause, and it is easy for many people to tell when they are overexerted. Resting, staying hydrated, and eating well can help the person recover.


High stress environments may cause a person to feel drained and lethargic. This may include having a physically or mentally demanding job or stressful living situation.

People with depression and anxiety disorders may also experience lethargy.


Some acute injuries may cause lethargy, as the body uses its energy to heal the injury.

For instance, people who break a bone may find that they are much less active and more lethargic as the bone heals.

A lack of exercise

Although a very intense workout may leave a person feeling drained or lethargic the next day, an overall lack of exercise may also cause similar symptoms.

People, especially older adults, who get little to no exercise may become easily winded or fatigued. Over time, everyday events may lead to overexertion and lethargy.

Drug or alcohol use

The use of certain substances — including cannabis, alcohol, and narcotic drugs — may also lead to feelings of lethargy.

This may be due to the way each individual drug acts in the body or other factors surrounding drug use, such as a lack of sleep or proper nutrition.

A lack of sleep

A temporary lack of sleep or difficulty getting to sleep may cause lethargy the next day. A person who does not sleep enough or has a severe lack of sleep may notice that they feel very sluggish and lethargic the next day.

Without proper rest, the body may not have enough time to recuperate, leading to lower energy levels the next day.

Too much sleep

On the other hand, oversleeping may also cause similar feelings of lethargy. This may have to do with the body’s natural sleep and wake cycle.

Sleeping too much may cause a disruption in these cycles, which may lead to lethargy over time.


Several medications may result in a lack of energy or feelings of indifference. Some people may be more sensitive to these effects.

Anyone taking a medication that causes them to experience lethargy as a side effect should talk to their doctor.

Some side effects are temporary as the body gets used to a medication. In other cases, the doctor may want to lower the dosage or suggest a change of medication.

Other underlying conditions and treatments

A number of underlying physical and mental health conditions may involve feelings of lethargy and fatigue, including:

Symptoms of lethargy involve feeling slow or sluggish in both the body and brain. More specifically, they may include:

  • decreased alertness
  • difficulty thinking
  • fatigue
  • low physical energy
  • sluggishness
  • a loss of motivation
  • changes in mood
  • an indifferent or apathetic attitude
  • brain fog
  • sleeping too much

Preventing lethargy may not be possible in every case, but in general, there are some things a person can do to have more energy and stay motivated each day.

These include:

  • getting enough sleep each night (but not oversleeping)
  • avoiding overexertion by controlling workouts and timing outdoor activities
  • eating a varied, well-balanced diet that contains plenty of fruits and vegetables
  • avoiding sugary foods that may cause an energy crash
  • avoiding high caffeine drinks and other energy drinks, which may cause a drop in energy after a few hours
  • going to bed at about the same time each night
  • winding down before bed, such as spending time away from screens and doing calming activities

Other tips for preventing lethargy will depend on a person’s lifestyle and risk factors. If lethargy is the result of an underlying health condition, taking measures to treat it will also help prevent feelings of lethargy.

Treatment for lethargy will depend on the underlying cause and can vary greatly from person to person.

Simple causes, such as overexertion, may only require resting, staying hydrated, and eating well.

Treatment for chronic lethargy linked to an underlying health condition will focus on treating the condition to see an improvement in symptoms.

Lethargy can be frustrating and leave a person feeling drained. However, it is not typically a sign that a person needs immediate medical attention.

Rest and proper nutrition can help a person recover from many cases of acute lethargy.

In the case of longer lasting lethargy, a person should see a doctor. There may be an underlying health condition that a doctor can help identify and treat.

It is a good idea to pay attention to other symptoms, too, as these may help the doctor identify potential causes and complications. This can lead to earlier treatment.

People who experience sudden, severe symptoms need immediate medical attention. Some concerning symptoms may include:

  • fainting
  • shortness of breath
  • pale, clammy skin
  • rapid breathing and heart rate
  • severe pain
  • slurred speech or difficulty speaking
  • difficulty controlling the facial muscles
  • difficulty moving the arms and legs

These may be symptoms of something serious.

If lethargy causes changes in mood that lead to suicidal thoughts, a person needs urgent medical care.

Lethargy refers to a state of fatigue and low energy.

A person experiencing lethargy may not have the energy or motivation to do the tasks they need to each day.

There are a number of possible causes of lethargy, ranging from occasional overexertion to underlying health conditions.

Anyone experiencing chronic symptoms of lethargy should talk to a doctor or healthcare provider for a full diagnosis and to discuss treatment options.