Water makes up at least two-thirds of the human body and plays an important role in your normal functions. Subsequently, dehydration can lead to a number of other ailments, from migraines¹ and constipation to kidney stones. It can also take a toll on your brain health, affecting your mood and overall brain function. For example, when you’re dehydrated, you’re more prone to irritability, anxiety and fatigue.

Dehydration happens when you’ve lost too much water in your body without replacing it, preventing your body from performing its normal functions. Mild dehydration can easily be treated but if it reaches extreme levels, it can be life-threatening and will require immediate medical attention.

Dehydration Shrinks Your Brain

About three-quarters of your brain is water, and when dehydrated, your brain actually shrinks in volume. (This shrinking is what causes a dehydration headache.)

Even mild or temporary dehydration can alter your brain function and impact your mood² as revealed in a 2013 study,³ in which 20 healthy women in their mid-20s were deprived of all beverages for 24 hours. While no clinical abnormalities were observed in the biological parameters (urine, blood and saliva), thirst and heart rate did increase and urine output was drastically reduced. As expected, the urine also became darker. As for mood effects, the authors noted:

“The significant effects of [fluid deprivation] on mood included decreased alertness and increased sleepiness, fatigue and confusion. The most consistent effects of mild dehydration on mood are on sleep/wake parameters…”

Fortunately, within 20 minutes of drinking some water, effects such as these are reversed. Dehydration-induced headaches are also rapidly alleviated once you rehydrate. Interestingly, cold water absorbs 20 percent faster than tepid water, so to increase the speed of recuperation, drink chilled water opposed to room temperature water.

If your kids struggle with frequent fatigue and mood swings, consider making sure they drink sufficient amounts of water. I’ll give you several tips for how to ascertain your hydration level below. Kids are at higher risk for dehydration for the simple fact that they’re more prone to drinking sweet drinks like soda and fruit juice instead of plain water.

A Harvard study^4,5,6 found more than half of American children are dehydrated, which can have repercussions for their health and academic performance. About one-quarter of children in the U.S. do not drink water on a daily basis. Overall, boys were 75 percent more likely to be inadequately hydrated than girls — a finding that dovetails with previous studies⁷ showing boys drink more sugary beverages than girls.⁸

How Dehydration Impacts Executive Brain Functions

Another study, published in 2011, found that dehydration caused by exercise-induced sweating led to “a significantly stronger increase in fronto-parietal blood-oxygen-level-dependent (BOLD) response during an executive function task … whereas cerebral perfusion during rest was not affected.”⁹

Because the increased BOLD response was not paralleled by diminishing cognitive performance, the researchers concluded that dehydration impaired metabolic activity in the brain, and that participants exerted a higher level of neuronal activity to maintain a normal performance level as a result.

“Given the limited availability of brain metabolic resources, these findings suggest that prolonged states of reduced water intake may adversely impact executive functions such as planning and visuo-spatial processing,” the researchers said.