Why does your day job make you so tired? For those who have office-based careers and spend the majority of the day sitting at a desk, doing no physical activity apart from getting up for a glass of water or stretching for our snacks, the 6pm sleepiness we feel can be akin to having spent the day hiking.

Is it a lack of physical movement that makes us sleepy? Potentially. But could it also be down to the fact that hard mental work is just as depleting on the body as physical work?

How much energy does the brain use?

Physical activity works up an appetite because “every muscle we move requires energy and generates metabolic processes that also require energy,” explains dietitian Sophie Medlin. “When we do physical activity or work, our body is expending energy in the movement of our muscles but also in the increased heart rate and breathing that is required when we exert ourselves.”

However, it isn’t just the muscles that are visibly working – your legs when you run or your biceps when you curl – that require energy. “Your brain thinking, your liver and kidneys filtering, and your bowel absorbing and digesting all require energy,” says Medlin.

“Energy in the body is in the form of calories, and every cell requires energy to do its job. As that energy is used up, it takes up more energy for its next job.” 

So that time you spend in deep focus, stressing over your job or simply doing a lot of mental work requires more fuel than you might think. “Your brain is actually your ‘hungriest’ organ. It is only 2% of our body mass but it uses 20% of our energy,” says Medlin.

“Because everything we do, including our heart beating and our lungs breathing, requires brain function. Our brain is using a lot of energy even when we’re sleeping. When we then ask our brain to do more work – typing, for example – it is using up more energy. When we’re stressed and anxious or are under a lot of pressure, or when we’re in complex social situations, our brain is working even harder.”

In a study by the University of Northumbria, participants who had to complete word or number tasks had sharper declines in blood glucose levels – a sign of energy expenditure – than those who had to complete a key-pressing task. Researchers concluded that “a period of intense cognitive processing […] may be linked to increased neural energy expenditure”. 

A bowl of fruit and cereal next to a laptop
Certain foods, like berries and oily fish, can help your brain during times of stress

It is those neurons that are responsible for our increasing energy needs when focusing. Neurons are the cells in our brain that transport information across the central nervous system. “They help us to connect ideas, retrieve memories and complete learned behaviours. These neurons are the cells in our brain that consume the most energy,” says Medlin.

These cells require 75-80% of the total 20% of energy the brain uses. “The more mentally stimulated we are, the greater the neuronal activity and the greater the energy we’re expending. In stressful situations, your brain is using a lot of energy and even in social situations where there is a lot of information to collect and process, your brain is busy and therefore using more energy,” explains Medlin.

“A good analogy is how quickly your laptop battery gets used up when you’re on a video call while secretly watching Netflix and messaging friends versus when you’re just typing and answering emails.”

How to feed a hungry brain

Our biology is smart – we naturally feel hungrier when we have been doing a lot of mental work, so eating to your appetite should give you the fuel you need to get through the extra load. “We also get a greater dopamine hit when we eat when we’re stressed, which means the vast majority of stressed people already consume more energy, particularly when it’s chronic,” says Medlin.

“What would be much more helpful than responding to those happy hormone hits is if we fed our brains with all the right things every day so we can manage stress better without our brain becoming exhausted.”

As the brain, like all organs in the body, relies on glucose, carbohydrates are crucial to power it. But Medlin also recommends brain-friendly foods such as oily fish, berries, green leafy vegetables, eggs and nuts.

Those high-fat foods are extra important, as 60% of our brain is fat and around 25% of that is an omega-3 fatty acid called DHA, usually found in fish or algae oil. “When we don’t consume enough oily fish or take algae oil supplements in the case of vegans, our brain can’t cope as well with day-to-day stress and pressure and we are likely to become mentally exhausted more quickly.

“There is great data to show that omega-3 supplementation has an amazing impact on behaviour in schools, reducing reoffending in prisoners and can improve working memory in older people. This means that anyone not consuming at least two portions of oily fish per week can expect benefits from supplementation,” says Medlin.

So who among us is guilty of not treating their work as nutritionally demanding? It’s not unheard of for desk-bound workers to skip breakfast or eat small portions due to having done ‘nothing’ all day. But it’s clear that we need to start respecting our mental hunger a bit more.

“Maybe it’s because we can’t see the brain in the same way we can other organs, but I also think people are less interested in energy consumption during intense mental work because we can’t relate it to burning calories in the same way we do physical activity – and that is the main reason that many people think about energy intake,” says Medlin. But for all the talk about pre- and post-workout nutrition, pre- and post-work food deserves the same amount of thought. 

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