The human brain is 75% water. It shrinks when we become dehydrated. How might this affect brain function, you may ask? A study was done taking a group of school children and randomly assigning them to drink either 250 ml or no water before taking a test. Which group do you think significantly did better? -the group given the water. The result of this research suggested that "even children in a state of mild dehydration, not induced by intentional water deprivation or by heat stress and living in a cold climate, can benefit from drinking more water and improve their cognitive performance."
Does dehydration affect mood also since our mood has a lot to do with brain activity. Absolutely, it does! How well hydrated we are affects our mood. The following have been shown to be results of poor water intake :
sleepiness and fatigue
lower levels of vgor and alertness
and increased feelings of confusion.