Daytime drowsiness can be greatly increased by dehydration since your body and brain require adequate fluids to function. Dehydration causes the blood volume to decrease, which lowers the amount of oxygen and nutrients that reach the brain. This results in weariness, headaches, difficulty concentrating, and sleepiness. Additionally, dehydration can impact electrolyte balance and blood pressure, causing weakness or drowsiness. Dehydration, even minor, can affect attentiveness. Having adequate water throughout the day often boosts vitality. A doctor may search for medical reasons if severe drowsiness persists after enough fluids and sleep; medications such as modvigil 200 are administered under medical care for some sleep problems.