Water accounts for 75% of brain mass  and it contributes to the maintenance of normal physical and cognitive functions. The body can cope with a 1% loss of water mass without impairing cognitive functions. Beyond 1% is categorised as dehydration.
Investigations into dehydration and cognitive function were first carried out in military populations where they were exposed to heat. The study found that losing 2% of your body weight fluid (around 4 cups) reduced short-term memory, working memory, and sustained attention . Fortunately, cognitive deficits can be quickly reversed by replenishing fluid levels in the body.
The University of Westminster that discovered consuming small amounts of fluid in 25ml water supplements facilitated visual attention and short-term memory . Perhaps more surprising were the findings of the University of East London as they were able to demonstrate a significant improvement in exam performance. 25% of 447 psychology students that took water with them into an exam scored an average of 5% higher marks . Another study found that men who completed 6 cognitive tasks whilst dehydrated had reduced attention to task and visual working memory response .
Does hydration status impact your anxiety? Possibly...
...Under exam conditions or when undertaking mentally strenuous tasks.“Water consumption may alleviate anxiety which is known to harm exam performance” , Dr Pawson, Head of Psychology at the University of East London. A study on a group of men that completed 6 cognitive tasks and a mood questionnaire whilst dehydrated also showed an increase in tension and anxiety during tasks  whilst investigations into dehydration in a rat study marked a reduction in GABA released . GABA is a chemical messenger that is associated with a relaxed and alert mental state.
Dehydration is often accompanied by an imbalance of essential minerals called electrolytes including sodium, potassium, and chloride. Electrolytes play an important role in facilitating the movement of nutrients across the blood-brain barrier to enter the brain .
How much water do you need to maintain cognitive performance?
• The NHS recommends that you drink between 6-8 glasses of water per day. Female adults should aim to drink 2 litres a day and male adults should aim to drink 2.5 litres a day.
• Frequency is important! According to the Harvard Medical School, “a general thumb for healthy people is to drink two to three cups of water per hour” . People had faster reaction times after drinking 500ml of water shortly before taking a reaction test .
• Restrict your fluid intake between 2- 4 hours before you go to sleep. Getting up in the night to empty your bladder can affect your sleep quality .
Is it possible to over hydrate?
Your desire to be healthy and hydrated is completely normal, but what happens if you unknowingly exceed your required intake? Overhydration is rare but can occur with a rapid rate of rehydration. The effects of overhydration were highlighted by the loss of a famous hockey player “Walker Wilbanks” in 2014.
Are some drinks better than others?
• Caffeinated beverages
The combination of caffeine and water has been clinically proven to boost mental performance. Following low (∼40mg) to moderate (∼300mg) caffeine doses mixed with water improves alertness, attention, and reaction time . A study by the University of Barcelona showed a classic espresso with 100mg caffeine has improved activity levels and alertness amongst adults after as little as 10 minutes . The effect from caffeine reaches peak levels within 30 to 60 minutes .
• Energy drinks
Energy drinks contain high levels of a stimulant, usually caffeine. Research showed that the consumption of energy drinks such as red bull can significantly improve alertness, reaction time, and concentration . Though be aware energy drinks can also obtain high levels of calories and sugar.
• Isotonic beverages
An isotonic beverage contains similar concentrations of salts and sugars as in the human body. They have a synergistic effect as they provide a quick replacement of fluid and electrolytes lost during exercise to avoid dehydration. The drinks dominate the fitness environment but watch out for high levels of glucose (sugar).
• Sugar-free beverages
Sugar-free drinks contain artificial sweeteners which have a similar reward feedback loop to sugars. They increase the activity of your reward and pleasure pathway in the brain which releases dopamine which can encourage you to crave more sugary foods . A study conducted at Yale University found that the consumption of sweetened beverages increased the risk of inattention, hyperactivity, and attention deficit hyperactivity disorder (ADHD) by 66% amongst healthy middle school students .
• Fruit infusions
If water is too plain for you, a genius hack for a healthier option is to infuse your favourite fruit water. You can even control the sweetness and sugar content of your drink by tailoring the number of fruits used and the duration you let it soak in the water.