Why High Performers Become Fatigued — And Why It’s Not (only) About Workload

High performers rarely collapse suddenly. More often, something happens first, slowly. Work continues, meetings are attended, decisions are made. From the outside, everything appears to function normally.

Yet sustaining attention requires a little more effort than before. By mid-afternoon mental clarity fades, tasks take longer, and the usual solutions - another coffee, a longer push through the afternoon - seem less effective. At that moment the explanation usually turns inward:

“I need more discipline.”

“I just need to push through.”

But biology suggests a different interpretation.

In many high-performing professionals, fatigue does not appear because the body cannot handle stress. It appears because the stress response never fully switches off.

When that happens, the systems that regulate energy begin to shift gradually. At first the changes are subtle. Over time they begin to affect the very capacities modern organizations rely on most: sustained attention, clear decision-making, and cognitive endurance.

Many leaders recognize this pattern - either in themselves or in members of their teams. The challenge is that the transition happens slowly enough to be normalized.

And that is precisely why understanding the biology of stress matters.

Stress Was Designed to Improve Performance

From a biological perspective, stress is not the enemy of performance. In fact, it is one of its foundations.

When the brain perceives a challenge, it activates a network known as the hypothalamic–pituitary–adrenal axis. Within seconds, hormones such as cortisol and adrenaline begin mobilizing energy. Glucose becomes more available to the brain, cardiovascular output increases, and attention sharpens.

For short periods of time, this system is remarkably effective.

Focus improves. Reaction times accelerate. Decision-making becomes faster. Many leaders are familiar with these moments when pressure seems to unlock unusually high productivity.

Stress, in this sense, is not the problem.

The difficulty begins when the system remains active long after the challenge has passed.

Because thinking - especially the kind of thinking required in modern leadership - is metabolically expensive. The brain represents only about 2% of body mass, yet it consumes roughly 20% of the body’s energy at rest (Raichle & Gusnard, 2002).

Sustained attention, emotional regulation, and complex decision-making all depend on a continuous supply of cellular energy.

When those energy systems remain stable, cognitive endurance follows naturally. When they begin to destabilize, the brain signals it quickly - often through fatigue, reduced concentration, or declining decision quality.

The Three Phases of Stress-Related Fatigue

Stress does not disrupt energy systems immediately. The body is remarkably good at adapting. Instead, the shift happens gradually as the body moves through different stages of physiological adaptation.

The earliest description of this process came from Hans Selye’s work on the General Adaptation Syndrome, later expanded through the concept of allostatic load - the cumulative biological cost of adapting to chronic stress.

Phase 1: Alarm — The Performance Surge

The first phase is familiar to anyone who has worked under pressure. When a challenge appears, the body mobilizes energy quickly. Cortisol and adrenaline increase glucose availability, the nervous system becomes more alert, and attention sharpens.

For a period of time, performance actually improves.

Many professionals experience this phase as a period of high output. Projects move forward quickly, decisions feel clear, and productivity rises.

What is easy to miss, however, is that this heightened performance comes with an increased metabolic cost. The brain consumes more energy, and the systems responsible for producing that energy must work harder to sustain it.

In the short term, this is perfectly manageable.

But the body was not designed to remain in this state indefinitely.

Phase 2: Resistance — The “Wired but Tired” State

If the pressure continues, the body attempts to adapt. Cortisol levels remain elevated, and the nervous system stays partially activated. Outwardly, individuals may still appear highly productive. Internally, however, the balance begins to shift.

Sleep becomes lighter or fragmented. Energy fluctuates throughout the day. Sustaining attention requires greater effort than before.

This stage is often described by professionals as feeling “wired but tired.”

At the cellular level, prolonged stress begins to influence mitochondrial function, the system responsible for producing ATP - the molecule that powers cellular energy. Research suggests that psychological stress can alter mitochondrial efficiency, affecting how effectively cells generate energy (Picard & McEwen, 2018).

Because neurons depend heavily on continuous energy supply, even modest changes in energy production can gradually reduce cognitive endurance.

Phase 3: Exhaustion — When Energy Regulation Breaks Down

If the stress response remains active long enough, the body eventually struggles to maintain its compensatory mechanisms. Researchers describe this stage as high allostatic load, when the cumulative burden of stress begins to disrupt multiple physiological systems.

Hormonal rhythms may become less stable. Inflammatory signaling increases. Mitochondrial energy production becomes less efficient. Sleep disturbances often intensify.

The result is a form of fatigue that no longer resolves easily with rest.

At this stage, individuals frequently report reduced concentration, slower decision-making, and diminished cognitive flexibility - precisely the capabilities modern leadership roles require most.

In biological terms, the body has shifted from sustaining performance to protecting its remaining energy.

A Different Way to Think About Fatigue

Fatigue in high-performing professionals is often interpreted as a problem of motivation or resilience. Biology suggests something different. In many cases the issue is not the amount of stress people experience, but how continuously the stress response remains activated. When the body cannot exit this state, the systems that regulate energy - hormonal rhythms, mitochondrial function, and sleep - begin to lose stability. The result is not simply tiredness, but a gradual decline in the biological capacity for sustained cognitive performance.

This insight shifts how we think about performance in modern organizations. If cognition depends on energy, and energy depends on biological regulation, then protecting those systems becomes a leadership challenge - not just an individual responsibility.

In a previous article I explored how productivity ultimately begins with metabolism and cellular energy. Stress adds an important dimension to that discussion: the body must not only generate energy, it must also recover it.

Which raises an important question for leaders and organizations:

Are we designing environments that continuously activate the stress response, or environments that allow the biological systems behind energy and cognition to reset?

Because long-term performance does not simply depend on how hard people work.

It depends on how well their energy systems are protected.

References

• McEwen BS & Wingfield JC (2003). The concept of allostasis in biology and biomedicine.

• Picard M & McEwen BS (2018). Psychological stress and mitochondria.

• Raichle ME & Gusnard DA (2002). Appraising the brain’s energy budget.

• Sapolsky RM (2004). Why Zebras Don’t Get Ulcers.

• Selye H (1956). The Stress of Life.