If you don’t stay hydrated, your physical performance can suffer.

This is particularly important during intense exercise or high heat.

Dehydration can have a noticeable effect if you lose as little as 2% of your body’s water content. However, it isn’t uncommon for athletes to lose as much as 6–10% of their water weight via sweat (1Trusted Source, 2Trusted Source).

This can lead to altered body temperature control, reduced motivation, and increased fatigue. It can also make exercise feel much more difficult, both physically and mentally (3).

Optimal hydration has been shown to prevent this from happening, and it may even reduce the oxidative stress that occurs during high intensity exercise. This isn’t surprising when you consider that muscle is about 80% water (4Trusted Source, 5Trusted Source).

If you exercise intensely and tend to sweat, staying hydrated can help you perform at your absolute best.

SUMMARYLosing as little as 2% of your body’s water content can significantly impair your physical performance.

Your brain is strongly influenced by your hydration status.

Studies show that even mild dehydration, such as the loss of 1–3% of body weight, can impair many aspects of brain function.

In a study in young women, researchers found that fluid loss of 1.4% after exercise impaired both mood and concentration. It also increased the frequency of headaches (6Trusted Source).

Many members of this same research team conducted a similar study in young men. They found that fluid loss of 1.6% was detrimental to working memory and increased feelings of anxiety and fatigue (7).

A fluid loss of 1–3% equals about 1.5–4.5 pounds (0.5–2 kg) of body weight loss for a person weighing 150 pounds (68 kg). This can easily occur through normal daily activities, let alone during exercise or high heat.

Many other studies, with subjects ranging from children to older adults, have shown that mild dehydration can impair mood, memory, and brain performance (8, 9Trusted Source, 10, 11Trusted Source, 12, 13).

SUMMARYMild dehydration (fluid loss of 1–3%) can impair energy levels, impair mood, and lead to major reductions in memory and brain performance.

Dehydration can trigger headaches and migraine in some individuals (14Trusted Source, 15Trusted Source).

Research has shown that a headache is one of the most common symptoms of dehydration. For example, a study in 393 people found that 40% of the participants experienced a headache as a result of dehydration (14Trusted Source).

What’s more, some studies have shown that drinking water can help relieve headaches in those who experience frequent headaches.

A study in 102 men found that drinking an additional 50.7 ounces (1.5 liters) of water per day resulted in significant improvements on the Migraine-Specific Quality of Life scale, a scoring system for migraine symptoms (16).

Plus, 47% of the men who drank more water reported headache improvement, while only 25% of the men in the control group reported this effect (16).

However, not all studies agree, and researchers have concluded that because of the lack of high-quality studies, more research is needed to confirm how increasing hydration may help improve headache symptoms and decrease headache frequency (17Trusted Source).