Feeling more tired than usual? Your brain uses a lot of energy.


Being tired from physical exercise is simple to understand: run hard or lift something heavy for a while and you’re going to become fatigued. Yet whether we are exercising or not, our brain is constantly working out. Despite this, we often underestimate the effect of mental fatigue and the consequences of its causes.

The brain drain

Our brain uses more energy than any of our other organs and boasts an enormous power-to-weight ratio. Despite accounting for only 2% of body weight, the brain clocks in at 20% of your resting metabolic rate: the rate at which your body burns energy when at complete rest.

To get the energy required to keep that big brain of yours firing on all cylinders requires the transportation of blood vessels. Water is required by every cell in the body and helps carry oxygen and nutrients via these blood vessels to the brain, which is 73% water.

A little goes a long way

Given the brains water composition, even a small amount of dehydration can have significant consequences. Mild dehydration causes part of the brain to swell, intensifying neural signaling that can contribute to temporary mental exhaustion.

Mild dehydration is considered around 2%. For most of us, thats about 1 litre (1/4 gallon) of water. To put that in perspective, when going for a hard run you could reach that level in about half an hour. Unfortunately, our bodies thirst mechanism isn’t very reliable around this point, with tests regularly showing that people can’t perceive that they are 1.5% dehydrated.

What happens next?

Researchers in the US have found that functions like attention, coordination and complex problem solving suffer the most from dehydration, increasing the likelihood of errors in attention-related tasks.

“We find that when people are mildly dehydrated they really don’t do as well on tasks that require complex processing or on tasks that require a lot of their attention,” says Mindy Millard-Stafford, director of the Exercise Physiology Laboratory at Georgia Institute of Technology (GIT).

Staying charged

About a third of brain function goes towards continuously retracing neural pathways for cell maintenance. A lot of that function is made up of electrical impulses which require charged sodium, calcium and potassium ions (electrolytes) in order to keep the brain tissue alive.

Without this ionic balance cells can become damaged, leading to strokes and other disorders. This makes hydration one critical element for supporting healthy brain function. So next time you go for caffeine or an energy drink, have a think about your water intake first… Your brain could be thirsty!


  1. Matthew T. Wittbrodt et al, Dehydration Impairs Cognitive Performance, Medicine & Science in Sports & Exercise (2018)
  2. Matthew T. Wittbrodt et al, Exercise‐heat stress with and without water replacement alters brain structures and impairs visuomotor performance, Journal of Physiology (2018)
  3. Aubrey, Allison. “Off Your Mental Game? You Could Be Mildly Dehydrated.” NPR, 30 July 2018
  4. Aubrey, Allison. “Heat Making You Lethargic? Research Shows It Can Slow Your Brain, Too.” NPR, 16 July 2018
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