Understanding PEM Triggers: What Can Cause Post-Exertional Malaise?

For people living with ME/CFS, Long COVID, or other conditions associated with post-exertional malaise (PEM), everyday activities can sometimes lead to a significant worsening of symptoms.

Understanding what triggers PEM is an important part of symptom management — and those triggers aren’t always limited to exercise.

What Is Post-Exertional Malaise?

The National Institute for Health and Care Excellence (NICE) describes post-exertional malaise as a worsening of symptoms that can occur after minimal cognitive, physical, emotional, or social activity, or after activity that a person was previously able to tolerate.

Symptoms can typically worsen 12 to 48 hours after an activity and may last for days or even weeks. In some cases, PEM can lead to a relapse.

PEM may also be referred to as post-exertional symptom exacerbation (PESE).

Importantly, PEM is not simply being tired after doing too much. The activities that trigger PEM can vary significantly from person to person and may include physical, cognitive, emotional, social, and sensory experiences.

Physical Activity

Physical activity is one of the more recognized potential triggers of PEM.

Depending on the severity of someone’s condition, a physical trigger could be something as simple as:

• Sitting up in bed
• Brushing your hair
• Brushing your teeth
• Showering
• Cooking
• Cleaning
• Taking a short walk

For someone with significant activity intolerance, an activity that appears minimal to another person can require substantial energy.

Cognitive or Mental Activity

PEM isn’t exclusively physical.

Activities requiring concentration, memory, or sustained mental effort can also potentially trigger symptoms. Examples include:

• Reading
• Writing
• Mental calculations
• Working on a computer
• Using a tablet
• Using a smartphone

This is one reason why activities such as working, studying, or prolonged screen time may need to be carefully managed.

Social Activity

Social interactions can also require energy.

For some people, PEM triggers may include:

• Talking with someone in person
• Phone conversations
• Video calls
• Online social interactions
• Spending extended periods around other people

Social activity can involve both cognitive and emotional demands, which may contribute to an individual’s overall energy expenditure.

Emotional Stress and PEM

Emotions may also play a role.

Experiences that create heightened emotions — including excitement, anger, frustration, fear, grief, or guilt — can potentially contribute to symptom exacerbation.

Stress can be particularly complex because it involves both emotional experiences and physiological responses within the body.

Sensory Experiences

Sensory input can also be challenging for some people experiencing PEM.

Potential sensitivities may include:

• Noise
• Bright light
• Touch
• Changes in weather
• Certain foods
• Certain medications

Sensory sensitivities themselves can be symptoms and may contribute to the overall load placed on an individual’s system.

There Is No One-Size-Fits-All PEM Trigger

One of the most important things to understand about PEM is that triggers are individual.

What causes symptom exacerbation for one person may be completely manageable for another.

That’s why identifying your own patterns can be an important part of managing ME/CFS, Long COVID, and other conditions associated with PEM.

Keeping track of activities, symptoms, delayed symptom changes, and recovery time may help patients and their healthcare teams recognize patterns.

Most importantly, PEM management should be individualized. The goal isn’t simply to push through symptoms or assume that more activity is always better. Understanding an individual’s limits and triggers can help guide a safer and more supportive approach to daily activities and rehabilitation.

If you experience a delayed worsening of symptoms after physical, cognitive, emotional, social, or sensory activity, consider discussing these symptoms with a knowledgeable healthcare professional.

Understanding PEM starts with recognizing that energy expenditure can take many different forms — and your body’s response may not happen immediately.

Reference: https://meassociation.org.uk/wp-content/uploads/2025/02/Research-review-PEM-DECEMBER-2024.pdf