Can You Really Be Tired from Thinking?

You're exhausted. Again.

And you can't figure out why.

You review the day: checked some emails, did a load of washing, watched TV. Nothing major. Nothing that should leave you feeling like you've run a marathon. Yet here you are at 6 PM, completely wiped out, barely able to think about making dinner.

Before COVID, you used to power through days like this without a second thought. Now? Your body seems to have a completely different operating system, and nobody gave you the manual.

Maybe you've started tracking which activities drain you most. Changing the bedding? That's a 60% drain-you know you'll need to rest afterward. Deciding what to do first in the morning? About 40%. Checking emails? Only 20%, surprisingly.

You're getting systematic about it. You're being careful. You're managing your physical activities.

So why are you still crashing?

The Energy Drain Nobody Mentions

Here's what most advice about chronic fatigue focuses on: physical activities. How much you walked. Whether you climbed stairs. How long you stood while cooking. What you lifted.

And that makes sense, right? Physical exertion drains energy. Everyone knows this.

But there's something critical almost no one mentions when they talk about energy management:

Mental activities consume real biological energy from the same limited pool.

Not metaphorical energy. Not "feeling mentally tired." Actual, measurable biological energy that depletes your body's available resources just as much as physical tasks do.

Think about a typical day where you "didn't do much." What were you actually doing?

Probably a lot of thinking. Worrying about things you need to do. Deciding what to tackle first. Going over problems in your head. Ruminating about those blood test results. Wondering if something's really wrong.

Here's what research on cognitive fatigue shows: when you engage in sustained mental effort-and yes, worry and rumination absolutely count as mental effort-your brain depletes neurotransmitters in the prefrontal cortex involved in attention and decision-making. Studies on mental fatigue demonstrate that prolonged cognitive activity adversely affects both psychological and physiological well-being, creating a measurable state of exhaustion.

When you spend two hours worrying about your health, you're not just "sitting there." You're running your car engine in neutral-burning fuel without going anywhere.

What Your Worrying Actually Costs You

This explains something you've probably noticed but couldn't quite name: you can feel genuinely exhausted after a day of worrying, even though you "did nothing."

Because you didn't do nothing. You did something extremely energy-intensive. You just couldn't see it happening.

Research on rumination and worry shows they're both associated with reduced cognitive resources. The constant mental preoccupation drains cognitive and energetic resources, making it difficult to concentrate and make decisions. One study found that daily rumination intensifies impairment of well-being and acts as a trigger for emotional exhaustion.

So when you rated "deciding what to do first" as 40% draining, you were onto something real. Decision-making isn't free. Worrying isn't free. Mental activity costs energy-you've just been trained to think it doesn't count because you're not moving.

Now here's the question that changes everything:

If mental activity uses real energy, and you have a limited amount of energy available each day, what happens when you calculate your total energy expenditure?

You're not just accounting for the washing (60%) and changing bedding (60%) and checking emails (20%).

You also need to account for the three hours you spent worrying about whether you'll ever feel normal again. The mental loops about what's wrong with your thyroid. The anxiety about whether you're just being lazy.

Suddenly, that "light" day looks very different.

Why This Isn't Weakness

And here's where this gets even more important for you specifically:

You've been wondering if maybe you're just making excuses. If maybe you should be trying harder. If maybe this is some kind of personal failure.

It's not.

Researchfrom the NIH found that 4.5% of people who had COVID-19 met diagnostic criteria for chronic fatigue syndrome, compared to only 0.6% of people who were never infected. That's a seven-fold increase.

Post-viral fatigue syndrome is a recognized medical condition with a biological basis. Your body's energy production systems may have been affected by the infection. Studies show that patients with post-viral fatigue exhibit reduced oxygen extraction capacity and impaired mitochondrial function-actual measurable changes in how your cells produce energy.

This isn't you being weak. This is your biology dealing with something real.

The Truth About Your Energy Limits

There's a concept in chronic fatigue research called the "energy envelope." Imagine you have a certain amount of available energy each day-your envelope. Research shows that when people with chronic fatigue stay within their energy envelope, they reduce symptom flares and can make modest gains in physical functioning over time.

But here's the critical piece almost everyone misses:

Both physical AND mental activities draw from the same envelope.

That car battery analogy you've been using? It's more accurate than you realized. Your body has limited energy. It gets depleted by physical tasks AND by mental activities. Changing bedding draws from the battery. So does spending two hours in an anxious thought spiral.

This is why you've been crashing on days that looked "light" on paper. You were only counting half your energy expenditure.

Why Pushing Through Makes It Worse

You've probably noticed this pattern: you push through your fatigue to get something done-maybe you change the bedding even though you're already tired-and then you're completely wrecked the next day. Sometimes you have to spend the whole following day on the sofa.

That crash has a name in medical literature: post-exertional malaise. It's a hallmark of post-viral fatigue syndrome.

Here's what's happening: when you exceed your energy envelope, your body doesn't just get tired-it triggers a symptom flare. The research on post-exertional malaise shows that patients experience reduced systemic oxygen extraction and oxidative phosphorylation capacity after physical activity, mediated by dysfunctions in mitochondrial capacities.

In plain English: pushing beyond your limits doesn't just tire you out. It temporarily impairs your body's ability to produce energy at all.

And because mental activity depletes the same energy pool, you can trigger post-exertional malaise without doing a single physical task. You can crash from a day of worry alone.

What If Some Activities Give Energy Back?

But here's something interesting you discovered: filing work doesn't drain you. It actually feels energizing.

What's different about filing compared to worrying?

Filing has a clear outcome. You can see the progress. And then it's done. It's structured, purposeful activity with a definite endpoint.

Worrying just goes in circles. It never finishes. You could worry about the same thing for hours and get nowhere. You're expending energy without any productive output-the engine running in neutral.

Research on activity pacing shows that structured, purposeful activities with clear endpoints tend to reduce fatigue and psychological distress, while rumination increases both. A meta-analysis of activity pacing interventions found they were effective at reducing fatigue (effect size -0.52), as well as reducing psychological distress and depression.

Some activities give you a sense of completion and accomplishment that partially offsets their energy cost. Other activities-particularly worry and rumination-offer pure energy drain with no psychological benefit.

How to Track What's Really Draining You

So what do you do with this information?

You've already started doing the most important thing: developing awareness of your energy expenditure patterns. That percentage rating system you created? That's you building interoceptive awareness-the ability to notice your internal body signals. Research shows this awareness is trainable and therapeutically valuable.

Now you need to expand what you're tracking.

If mental activity uses real energy, and you have a limited envelope, then worry time needs to go on your energy budget just like changing bedding does.

Here's a small experiment to try: track your worrying and rumination time like you tracked your other activities. Give it a percentage rating. How draining does two hours of health anxiety feel? What about going over your to-do list in your head for the third time?

Then notice: are the days you worry less the days you have more energy for other things?

You're becoming a scientist of your own energy system. You're gathering data. You're learning to drive your car at the speed that works for your current engine, rather than redlining it and breaking down.

This isn't giving up. This isn't accepting limitation.

If you had a broken leg, you wouldn't try to run a marathon. You'd work with your body's current reality while it healed. That's not weakness-it's wisdom.

Your body is dealing with something real. The evidence-based approach is to work with your energy envelope rather than fighting it. Studies show that activity pacing reduces fatigue, distress, and depression in people with post-viral fatigue syndrome.

You're learning to recognize that mental activities have costs. That invisible energy expenditures are still real. That your exhaustion makes sense when you account for everything that's actually depleting your limited energy reserves.

What Changes When You See It

Once you see this, you can't unsee it.

Every time you catch yourself spiraling into worry, you'll recognize it for what it is: you're spending energy. Real energy. Energy that could go toward filing work that makes you feel accomplished. Or toward a physical task you actually need to complete. Or toward having a conversation with someone you care about.

You start making different choices. Not because you're forcing yourself, but because you understand the real cost.

"Do I want to spend 40% of my energy deciding what to do first, or can I just pick something and start?"

"I've been worrying about this for an hour. That's energy I'm burning. What else could I do with that energy?"

You're not broken. You're not lazy. You're managing a medical condition with a biological basis.

And the first step in managing it effectively is seeing the whole picture-including the energy drains that were invisible before.

What's Next

You now understand that worrying and rumination consume real energy from your limited daily envelope. You can see why you've been crashing on days that looked "light" when you only counted physical activities.

But knowing that worry drains energy raises an obvious question:

How do you actually reduce it?

You said it yourself: "It's not like I choose to do it."

Worry isn't usually a conscious decision. It feels automatic. Unstoppable. Like it happens to you rather than something you're doing.

If mental activity is depleting your energy envelope without your conscious choice, what can you do about it?

That's a skill worth developing. And it's exactly what we'll explore next.