Exercise physiologists at Georgia Institute of Technology found that dehydration impairs cognitive function, according to a summary of their work at Neuroscience News.  The brain itself also changes shape, with ventricles in the brain shrinking during exertion combined with heat, and expanding when dehydrated in addition to exertion and heat. Researchers did not believe that the shifting shape of the brain is what caused changes in cognitive function, however.  The study was recently published in the journal Physiological Reports, and the Neuroscience News summary can be found here:  https://neurosciencenews.com/dehydration-brain-shape-activity-performance-9723/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+neuroscience-rss-feeds-neuroscience-news+%28Neuroscience+News+Updates%29