The brain inflates slightly from drinking water opening certain restricted blood vessels and making it easier for oxygen to flow into certain areas of the brain. I feel a lot more awake after drinking a cup or two of cold water. People yawn apparently because there's a lack of oxygen in the brain, so this makes sense with how it remedies tiredness. Don't drink too much electrolyte water/juice/milk it doesn't work as well because it's salty and may dehydrate the brain, sucking water from your cells. If you drink a lot of water after a few cups, you'll need electrolytes though like Gatorade or coconut water, or you'll die.