It's easy to overlook, but making sure we drink enough water is often quite low on our priority list.
But are we putting ourselves at grave risk by doing so?
"Your immune system relies on water," says Dr Marilyn Glenville, author of Natural Alternatives to Sugar. "It carries nutrients to the cells, carries waste and toxins away from the cells and out of the body, keeps body temperature stable, protects joints and keeps the lining of your mouth hydrated and moist, reducing susceptibility to colds.
"Drinking plenty of water will also flush out waste and dilute mucus, helping eliminate bacteria and viruses that can make you ill."
And here are the surprising side effects of forgetting to drink water.
1. You are feeling tired
Our bodily fluids are comprised of water so when you get dehydrated, your blood becomes thicker and your heart has to use more energy to keep oxygen and other nutrients moving through the circulatory system.
"The body's way of conserving energy as it may not be functioning as efficiently in a hypo-hydrated state," says Dr Emma Derbyshire of Natural Hydration Council.
The Natural Hydration Council says one in 10 cases of tiredness are attributed to dehydration.
2. You are having digestive problems
"We all know that fibre is an essential nutrient in the digestive process, but without enough water, it can easily lead to constipation," says Dr Adam Simon, chief medical officer at PushDoctor.co.uk.
"Simply put, water makes it easier for you to go the toilet, softening your poo so that it's easier to pass.
"Some studies have also claimed that not drinking enough water makes it more likely that you'll develop stomach ulcers."
3. Your eyes are sore
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