Most of us know hydration matters. We’ve all heard the advice: drink more water, carry a bottle, aim for eight glasses a day. And yet, despite this awareness, many of us are still unknowingly walking around dehydrated. According to a recent report by Phizz, the disconnect is striking. Around 97% of people understand that hydration is important for health, yet 32% admit they can go an entire day without drinking water. Even more tellingly, 35% of people say they feel dehydrated every single day.

These numbers suggest that while the importance of hydration is widely recognised, our daily habits don’t always reflect that knowledge. And the consequences may be more significant than many people realise.

To better understand the real impact of dehydration, I spoke to Dr.Paul Anastasiades, a PhD neuroscientist and senior lecturer at the University of Bristol who is also a Co-Founder and helped develop the hydration science behind Phizz’s electrolyte products.

What emerges from the conversation is a fascinating insight into how something as simple as hydration can affect everything from brain function and stress levels to immunity and heart health.

Why Dehydration Affects the Brain First

One reason dehydration can feel so draining is because of how heavily the brain relies on water.

“The brain actually contains a very high proportion of water, about 80%,” explains Anastasiades. “So if you become dehydrated, it’s going to impact a whole host of bodily functions, particularly those involving cognition and mood.”

When hydration levels drop, the body moves outside its optimal physiological balance, something scientists refer to as homeostasis. In response, the body activates a stress response and releases the hormone cortisol.

“Your body likes to exist in this sort of ‘Goldilocks zone’,” Anastasiades says. “Not too hot, not too cold, not too hydrated, not too dehydrated. Anything outside that balance creates stress in the system.”

That stress can directly affect the prefrontal cortex, the part of the brain responsible for decision-making, focus and attention. At the same time, other areas of the brain involved in emotional regulation, including the amygdala, can become more reactive. The result? Brain fog, irritability and reduced mental performance.

“You’ve probably heard the term hangry when someone is hungry and their mood changes,” he says. “There’s actually a similar phenomenon with dehydration. Technically it’s sometimes referred to as being ‘thangry’, thirsty and angry at the same time.” It may not be a widely used term (yet!), but the underlying science is clear: dehydration can alter mood, concentration and cognitive performance long before we consciously realise we need water.

The Hidden Signs of Dehydration

Most people associate dehydration with intense thirst, but that’s often a late signal. In fact, many of the most common symptoms appear earlier and are easy to misinterpret. Headaches are one of the most frequent indicators. Reduced fluid levels can affect blood circulation and the balance of fluids surrounding the brain, triggering discomfort.

Fatigue is another major sign. When the body is dehydrated, blood volume drops slightly, meaning the heart must work harder to pump oxygen and nutrients around the body. There’s also an interesting psychological effect that researchers have identified: dehydration can make everyday tasks feel harder.

“If you ask someone to complete a task, either a physical one like running on a treadmill, or a cognitive one that requires concentration, dehydrated people consistently report that it felt more difficult,” says Anastasiades. Scientists call this increased perceived effort.

“Life just feels like more of a slog,” he says. “Your body and brain have to work harder to do the same things.” Other subtle signals can include digestive issues, dry skin, reduced focus and irritability, all symptoms many of us might blame on a busy day rather than our hydration levels.

Why Hydration Matters for Immunity

One of the most surprising effects of dehydration lies in the immune system. Inside our respiratory system, in the nose, mouth and lungs, is a protective mucosal lining that acts as a first barrier against infection. This lining produces mucus that traps invading bacteria and viruses.

Tiny hair-like structures called cilia then sweep the mucus down toward the stomach, where stomach acid destroys the pathogens. But dehydration disrupts this protective system. “When you become dehydrated, the mucus becomes thicker and less mobile,” explains Anastasiades. “That means pathogens remain in contact with those tissues for longer, which increases the likelihood they’ll enter the body.” In other words, hydration helps maintain one of the body’s most important natural defence mechanisms.

The Surprising Impact on the Heart and Digestion

The effects of dehydration don’t stop at the brain or immune system. “Another surprising one is the impact on the heart,” says Anastasiades. When the body loses water, blood becomes more concentrated and thicker. As a result, the heart must work harder to circulate it.

“For every 1% level of dehydration, your heart rate increases by about five beats per minute,” he explains. Over the course of a day, that added strain can place noticeable stress on the cardiovascular system. Digestion can also be affected. Water plays an essential role in breaking down food and transporting nutrients through the digestive system. When fluids are limited, the body prioritises more critical systems like circulation and temperature regulation.

“If your body is dehydrated, it’s going to prioritise water for those vital systems,” Anastasiades says. “Digestion can suffer as a result.”

In more severe cases this can lead to constipation, reduced nutrient absorption and digestive discomfort.

Why Electrolytes Matter

One of the biggest misunderstandings about hydration is the idea that water alone is always enough. In reality, the body requires a balance between water and electrolytes, minerals such as sodium, potassium and magnesium that help regulate fluid movement.

“Hydration isn’t just about water,” says Anastasiades. “It’s about the balance between water and electrolytes.” These minerals help control how fluid moves into and out of cells, and they also enable the body to absorb water efficiently through the digestive system. A key mechanism involved is known as the sodium-glucose co-transporter. “For every two sodium molecules and one glucose molecule that your body absorbs, around 260 water molecules move into the body at the same time,” Anastasiades explains.

This mechanism forms the basis of many medical rehydration solutions and sports drinks. Products such as Phizz are designed using this principle, combining electrolytes and glucose in carefully balanced amounts to help the body absorb water more effectively than water alone.

Hydration Is Simple, But Often Overlooked

Given the widespread understanding of hydration’s importance, the statistics from the Phizz report raise an obvious question: why do so many people still fall short? Part of the problem is that hydration tends to be reactive rather than proactive. People often wait until they feel thirsty, or even unwell, before drinking enough fluids. But thirst is already a signal that the body is playing catch-up.

“Prevention is always better than cure,” says Anastasiades. “If you can build hydration into your daily routine, especially early in the day, you’re much less likely to end up feeling depleted later.”

In fact, he believes hydration may be the simplest health habit to improve. “When you think about the pillars of health; sleep, exercise, diet and hydration, hydration is arguably the easiest one to address,” he says. “It’s simple, accessible, and it has no negative side effects.”

A Small Habit That Can Make a Big Difference

The takeaway from the research is clear: hydration is fundamental to how our bodies and brains function, yet many of us still underestimate its importance in everyday life. With 35% of people reporting daily dehydration and a third able to go a whole day without drinking water, it’s a reminder that awareness alone doesn’t always translate into action. By paying attention to subtle symptoms, fatigue, brain fog, headaches or irritability and by understanding the role electrolytes play in fluid balance, it becomes easier to stay ahead of dehydration rather than constantly catching up. Because sometimes the difference between feeling energised and feeling exhausted isn’t a complicated health intervention. It’s simply remembering to hydrate.