Although there's no evidence to suggest that 8 glassed is "ideal," many studies show that even drinking a single glass before an exam or difficult task can improve performance. In fact, even mild dehydration can have a negative effect on cognitive performance.
A 2013 university study explored how hydration would affect people in various test-taking scenarios. On one morning 34 adults were given a variety of memory, attention, learning and reaction-time assessments. They were given a glass of water and a cereal bar for breakfast beforehand. On another morning, they only got a cereal bar. With water, participants’ performance on most tests were up to 14 percent shorter — especially for those who had reported feeling thirsty before the tests.
“Our studies so far would suggest that it could be helpful to have a drink before taking part in something that required quick reaction times,” said Caroline Edmonds.
However, she also discovered that water also made some cognitive tasks worse. Several studies have also shown that young children who drink a glass of water before a test do better on certain types of cognitive functioning.
Water can also perk you up and dehydration can cause you to feel fatigued. It forces your brain to work harder than it would otherwise to perform the same tasks, explains Matthew J. Kempton at King’s College London. Cold water, taken before a meal, may help curb your appetite slightly, according to a study in the Journal of Clinical Endocrinology & Metabolism finds. And when a National Institutes of Health–funded study, conducted by Gary Fraser, tracked 34,000 people for 14 years, it found that men who downed five to six glasses of water a day were nearly 70 percent less likely to die of a heart attack.
Research has shown that water supplementation positively affects cognitive performance in children and adults. The present study considered whether this could be a result of expectancies that individuals have about the effects of water on cognition. Forty-seven participants were recruited and told the study was examining the effects of repeated testing on cognitive performance. They were assigned either to a condition in which positive expectancies about the effects of drinking water were induced, or a control condition in which no expectancies were induced. Within these groups, approximately half were given a drink of water, while the remainder were not. Performance on a thirst scale, letter cancellation, digit span forwards and backwards and a simple reaction time task was assessed at baseline (before the drink) and 20 min and 40 min after water consumption. Effects of water, but not expectancy, were found on subjective thirst ratings and letter cancellation task performance, but not on digit span or reaction time. This suggests that waterconsumption effects on letter cancellation are due to the physiological effects of water, rather than expectancies about the effects of drinking water.
While dehydration has well-documented negative effects on adult cognition, there is little research on hydration and cognitive performance in children. We investigated whether having a drink of water improved children's performance on cognitive tasks. Fifty-eight children aged 7-9 years old were randomly allocated to a group that received additional water or a group that did not. Results showed that children who drank additional water rated themselves as significantly less thirsty than the comparison group (p=0.002), and they performed better on visual attention tasks (letter cancellation, p=0.02; spot the difference memory tasks, ps=0.019 and 0.014).
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