Water is one of the first and most important things. The brain just like the body needs water in order to perform, as we want it to. It has been proven now over time that when we are parched it will affect our brains. In fact, dehydration will lead to a rise is stress hormones. Physically this can manifest as lethargy, headaches, brain fog, and afternoon fatigue. Other manifestations of dehydration are lack of ability to focus, impaired short-term and long-term memory. It is best to drink approximately 3 liters a day or half your weight in ounces each day.