Your eyes will become dry, irritated and bloodshot if they don't get enough water.
"That's because they're usually protected by a thin film of moisture which, like your tears, contains water and salt," says Dr Simon.
"If the salt isn't sufficiently diluted, your eyes will feel it. This lack of moisture will also make life particularly miserable for people who wear contact lenses."
4. You have an increased risk of kidney stones
"Dehydration can lead to the formation of kidney stones," says Dr Simon.
"While most are small and can be passed out by the body, if kidney stones are large enough, they can block the urinary tract and cause stomach or groin pain.
"If you become severely dehydrated, there's a chance you could develop acute renal failure."
5. You have bad breath
"Keeping hydrated is essential for general wellbeing, but you may be surprised to know it is also essential to keep your breath in check too," says Luke Thorley, of Welling Corner Dental Practice and brand ambassador for CB12.
"Regular glasses of water help to maintain saliva flow, which washes away the âbad bacteria' that cause unpleasant breath."
6. You have aching joints
Joints need fluid to stay lubricated in order to function.
"It's important for your joints to stay lubricated in order for them to work properly," says Dr Simon.
"Getting plenty of fluids will help keep cartilage soft, which reduces your chances of developing arthritis or other joint problems."
7. You have a headache
"The loss of even 1-2% of body weight - equating to 500 ml to two litres - can bring on headache symptoms," says Dr Carrie Ruxton, a member of the Tea Advisory Panel.
"Alongside this, you may feel dizzy or lethargic. Start replacing fluid straight away if a headache comes on by sipping water steadily."
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