The simple way to banish brain fog

Ever had one of those of those days where your brain just won't co-operate? Believe it or not, you might just need a glass (or six) of water.

About two thirds of the adult human body is water, external. The brain, however, is about 73%, external – and studies have found even mild dehydration can impair cognitive performance, external to varying degrees.

This is sometimes known as 'brain fog' and can impact attentiveness, short-term memory and even your decision making ability, external.

"None of the cells in our body can work optimally when we're dehydrated," says dietitian Sophie Medlin. "The water in our body needs to constantly be changed because we use it to flush out toxins and support in-cell processes."

Most people should drink six to eight glasses of fluid a day, external. Water, lower-fat milks, lower-sugar or sugar-free drinks all count, as does tea and coffee – in moderation.

Depending on your diet, you also get some fluid from the water in your food, which will count towards your intake.

Your body regularly loses water through breathing, sweating and urination, but if the water isn't replaced at the same rate as it's lost, you become dehydrated.

The colour of your pee is a great indicator of how hydrated you are. It contains a waste product from your kidneys called urobilin that gets diluted the more fluid you drink.

Most of us should aim to drink enough so that our pee is a clear pale yellow colour.

Remember you can get dehydrated more easily if you have, external:

Older people might also get dehydrated, external more easily because of health conditions, medications and a lower thirst response, which happens as we age.

Getting rid of alcohol in our body uses a lot of water, says Medlin. So anyone concerned about brain fog does not need an additional hangover.

"Alcohol is a toxin and in order for our bodies to eliminate it, we first dilute it in our blood by diverting water into our bloodstream from other tissues," says Medlin. "Our liver then kicks in to process, neutralise and excrete the alcohol through our kidneys."

Fizzy beverages can speed up the absorption of alcohol into your body, so it's best to steer clear when you're drinking. Bonus points if you also drink a glass of water before going to sleep.

Too much caffeine can also cause dehydration because it's a diuretic. It increases blood flow to your kidneys, making you pee more.

But some research suggests coffee, external can be as hydrating as water if drunk in moderation. Medlin suggests having a glass of water alongside caffeinated drinks to combat any potential diuretic effects.

Sip water

Experts say adults should drink about two litres of fluid a day, external – but it may be better to sip than gulp it, external.

"Ideally, we will keep hydrated through the day by having a drink on the go at all times," says Medlin.

"When we consume a large volume all at once, our body has to adjust for this. The electrolyte shifts that happen can mean we lose more of that water in our urine than we would if we had drunk it slowly."

Think of your reusable water bottle as your hydration sidekick.

A glass at mealtimes

It might seem obvious, but another simple way to stay hydrated without much effort is at meal times.

Simply drink a glass of water at breakfast, lunch and dinner and you're halfway there.

Get at least five a day

Many foods, especially fruits and vegetables, contain water, external and contribute to your daily water intake.

Melons, lettuce, and cooked squash are between 90–99% water, while oranges, broccoli and carrots are 80–89% water, external.

High-salt foods like sausages, cheese and crisps, on the other hand, may dehydrate you as the body needs water to remove the salt.

Make water tasty

If you're not used to drinking plain water, you can jazz it up with natural flavourings.

Chef Donal Skehen adds mint, ginger, cucumber, citrus fruit, frozen fruits and a dash of fruit juice to his water to introduce some flavour.

But it's best to avoid sweetened drinks and straight-up juice.

"Drinks containing sugar may give a quick energy high, which mentally alerts you, but this is followed by sugar dip which leads to an energy low and mental tiredness," says nutritionist Sonal Shah.

A number of studies have found that drinking enough fluid to avoid mild dehydration helps support brain function, external.

But most experts tend to agree there's no benefit to drinking more than we need, so stick to six to eight glasses a day. Your body – and brain – will thank you.

Originally published November 2021. Updated March 2026.

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