It’s 8 p.m. You’ve just flopped on the couch after a long day. Maybe you stared at spreadsheets, negotiated with toddlers, or endured an endless Zoom call. Now, as you zone out to the soft flicker of a screen, it feels like your brain is finally off duty.

But it isn’t.

Deep inside your skull, your brain is still burning energy at nearly full throttle. According to new research, even when you’re not consciously thinking, your brain hums along like a high-performance engine idling — devouring glucose, managing your body, and rehearsing the future.

And when you do think hard? That burns only about 5% more energy.

This surprising finding, from neuroscientist Sharna Jamadar and her colleagues at Monash University, upends a common assumption — that mental effort guzzles energy while mental rest lets the brain off the hook.

Even complex cognitive tasks only incrementally increase the brain’s metabolic cost, the researchers found after analyzing decades of research on brain metabolism.

The Brain’s Base Load

The human brain makes up only 2% of our body weight. Yet it burns through 20% of our daily energy budget. That’s around 0.3 kilowatt hours a day — more than 100 times what your phone consumes. This energy is primarily used in the form of adenosine triphosphate, or ATP, the molecule that fuels brain cells.

Where does all that energy go?

A large portion powers the brain’s “housekeeping” duties. Neurons must maintain delicate electrical gradients called membrane potentials — voltage differences that keep them primed to fire. That alone may consume over half of the brain’s energy budget.

Even when we’re doing nothing, the brain is doing a lot. Much of that ongoing cost goes to what scientists call the default mode network — regions that fire up when you’re not focused on a specific task. It’s the circuitry of daydreams, memories, self-reflection, and benign emotional processing.

But even more vital is the brain’s role in running your body. While there is this sentiment that the brain is solely for thinking, a lot of brain power is expended on managing your body and the complex relationship between the various organs and tissues.

Mental Math and Metabolic Myths

Many of us feel drained after a tough mental workout. But that fatigue doesn’t come from the brain running out of energy. “It’s not because you don’t have the calories to pay for it,” Zahid Padamsey, a neuroscientist at Weill Cornell Medicine–Qatar, told Quanta. “It is because we have evolved to be very stingy systems.”

Our ancestors evolved in food-scarce environments, so our brains developed a kind of energetic frugality. That extra 5% boost in energy — just enough to engage your visual cortex while scanning a bus schedule — is small. But, across days, it adds up. In the evolutionary past, it might have meant the difference between survival and starvation.

Jamadar’s analysis found that even performing mentally demanding tasks — like memorizing word lists or solving logic puzzles — doesn’t cause a dramatic spike in glucose use. Rather, the brain shifts energy use regionally, the new meta-analysis (study of other previously published studies) found. When one network ramps up, others dial down. Paying attention to visual stimuli, for example, may suppress activity in the auditory cortex, leading to a kind of energetic balance.

“Our brain makes trade-offs when we focus on different things,” noted a recent article in The Conversation. It’s less of a global energy surge and more of a careful internal reallocation.

The Predictive Organ

More than a reaction engine, the brain is a prediction machine. It’s constantly running simulations — about the environment, your internal state, and what might happen next. This predictive coding minimizes energy use. Instead of responding to every new sensation from scratch, the brain compares inputs to internal models and updates only when there’s a mismatch.

Remarkably, the human brain does all this on just 17 watts of power — about the output of a dim light bulb. A supercomputer doing similar tasks would require megawatts of power.

That efficiency comes at a cost. The brain prioritizes parsimony over speed. Neurons could fire at rates of up to 500 times per second. But on average, they fire at a poky 4 hertz (4 cycles per second). Why? Because each signal comes with a metabolic price tag. “We have evolved to maximize information transmission per ATP spent,” said Padamsey.

In practical terms, the brain sacrifices redundancy for thrift. Many synaptic transmissions fail. But overall, enough information gets through to guide behavior — while staying within the budget.

How We Measure Mental Energy

So, how do scientists know all this?

They can’t just peer into living human neurons with a microscope. Instead, they use tools like PET scans and fMRI to estimate how much glucose or oxygen different brain areas are using. For instance, [¹⁸F]-fluorodeoxyglucose PET captures how a glucose analog gets trapped at active synapses. fMRI, meanwhile, relies on changes in blood oxygen levels to track activity.

Both methods are indirect. But when used together, they paint a compelling picture: even when resting, the brain’s regions flicker with coordinated activity. And when a person shifts to an active task, glucose metabolism increases — but only modestly, and mostly in the task-relevant areas.

Previous research showed how measuring oxygen via CO₂ output during tasks like mental arithmetic confirms small but measurable boosts in consumption. Yet these increases may also reflect stress-related arousal, not just pure cognitive effort.

MRI studies have shown that watching a moving object versus passively observing it raises activity in the visual cortex by just 1%. So even when you’re “thinking hard,” you’re not demanding much more energy than when you’re casually staring out the window.

So, Why Do We Feel Drained?

If thinking doesn’t demand much extra energy, why do mentally tough days feel so exhausting?

The answer lies not in glucose but most likely in stress. Challenging tasks often come with emotional stakes — frustration, uncertainty, performance anxiety. These engage the sympathetic nervous system, flooding the body with cortisol and other stress hormones. That cascade can leave us feeling tired, foggy, or even physically spent.

Moreover, tasks that require sustained attention or decision-making often trigger mental fatigue, a phenomenon still not fully understood. One theory is that fatigue is the brain’s way of nudging us to stop before the costs outweigh the benefits. “You’re going to activate fatigue mechanisms that prevent further burn rates,” said Padamsey.

In other words, your brain is regulating not just what you do, but how long you do it.

Energy and Aging

As we age, that delicate energy balance becomes harder to maintain. Cerebral glucose metabolism declines gradually in the frontal and temporal lobes — areas tied to memory and decision-making. In Alzheimer’s and related conditions, this slowdown appears early, sometimes years before symptoms.

These regions, often “connector hubs” in the brain’s network, are among the most energy-demanding. They coordinate communication between far-flung brain areas. And their high metabolic rates may make them more vulnerable to energy deficits and degeneration.

Insulin resistance, common in aging and diabetes, further impairs glucose delivery to the brain. This can lead to cortical thinning, impaired memory, and increased risk of dementia. The research points to one clear message: preserving metabolic health may be crucial for maintaining cognitive function.

The Brain, Reimagined

The brain is a thrifty, dynamic, and astonishingly efficient organ — capable of regulating your heartbeat while solving calculus and wondering what to eat for dinner.

Even when you feel idle, it is orchestrating many background tasks: keeping your body in balance, filtering your thoughts, preparing for what’s next. And when you do engage in focused mental labor, the metabolic costs are surprisingly modest — but the subjective toll can still be real.

So, the next time you feel mentally wiped after a hard day’s thinking, remember: your brain isn’t out of gas. It’s just enforcing a deeply evolved rule of biological efficiency. It has to. Because it’s always on.

The new review of the brain’s metabolic cost appeared in the journal Trends in Cognitive Science.