There are days when, even without doing any physical labor, we feel as though we’ve been carrying stones all day. A heaviness settles—not on the body, but on the mind. Long hours in front of screens, endless attention-demanding tasks, the constant effort to think about several things at once… By evening, we often find ourselves saying, “I didn’t do anything today, yet I’m exhausted.” In reality, this is the brain’s cry for help: “My energy is depleted.”
The brain accounts for only about two percent of body weight, yet consumes nearly twenty percent of the body’s total energy. This energy—primarily in the form of glucose—fuels neuronal communication.
Tasks that require prolonged focus—writing reports, preparing presentations, strategic planning, or emotionally charged conversations—gradually drain these energy reserves. As they diminish, attention wanes, thinking becomes foggy, and even simple decisions begin to feel difficult. This is not laziness; it is the brain’s protective response. Much like an overheating device that slows itself down, the brain reduces performance to prevent damage.
At the core of mental fatigue often lies a persistently activated nervous system. Cortisol—the primary stress hormone—keeps us alert in the short term. Yet, under continuous pressure, anxiety, and urgency, cortisol levels remain chronically elevated. The brain then remains in a constant state of alarm, even in the absence of real danger. A single email notification, a message sound, or the thought of tomorrow’s meeting can trigger this response. Over time, the prefrontal cortex—the region responsible for reasoning, planning, and self-control—begins to shut down. The amygdala takes over, shifting the brain from a reflective to a reactive state: the “fight or flight” mode.
The Paradox Of Rest Without Rest
Another curious feature of mental fatigue is the inability to truly rest, even while doing nothing. We lie in bed, but the mind refuses to quiet down; the body may be tired, yet thought continues. This is when the brain’s default mode network comes into play—a system that remains active when the mind is idle, revisiting the past and projecting into the future. When this network becomes overactive, mental energy continues to be consumed even in stillness. In short, rest is not merely about stopping; it is about allowing the brain itself to become silent.
Mental Fatigue In Everyday Life
Checking notifications on the way to work, reading emails during breakfast, talking while thinking about the next task—each of these habits gradually depletes the brain’s energy budget. By evening, we often wonder why nothing feels enjoyable anymore. Throughout the day, the mind must repeatedly reorient its attention. Each redirection costs a small amount of energy. Accumulated over hours, these micro-depletions leave us exhausted, even when little seems to have been accomplished.
The simplest way to counter mental fatigue is to respect the brain’s natural rhythm. Being fully present in what we do—thinking only of what we’re engaged in, not what comes next—helps minimize unnecessary energy loss. Short walks, breathing exercises, or simply gazing at nature are not clichés; they truly slow neural circuits down. EEG studies have shown that viewing natural scenes for just a few minutes increases alpha wave activity—associated with relaxation and mental restoration. Sometimes, standing by a window and quietly observing the world outside is enough to activate the brain’s own repair mechanisms.
Mental fatigue syndrome has quietly become one of the invisible epidemics of modern life. Yet this “endless load” stems not only from external demands but also from our internal pressures—the relentless drive to produce, to achieve, to keep up. In doing so, we deprive the brain of its right to rest. Perhaps productivity today should not be measured by how much we do, but by how presently we do it. Rest, after all, is not a luxury for the mind—it is a biological necessity.
References
Arnsten, A. F. T. (2009). Stress signalling pathways that impair prefrontal cortex structure and function. Nature Reviews Neuroscience.
Raichle, M. E. (2015). The Brain’s Default Mode Network. Annual Review of Neuroscience.
McEwen, B. S. (2017). Neurobiological and systemic effects of chronic stress. Nature Medicine.