Even mild dehydration can drain your energy and make you tired.

 https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/water/art-20044256

• About 15.5 cups (3.7 liters) of fluids a day for men 7.8 pints
  
• About 11.5 cups (2.7 liters) of fluids a day for women 5.7 pints