1. Dehydration 
Being dehydrated is one of the scariest causes of being tired all the time. Because dehydration isn’t always easily detected, it can easily sneak up on you if you’re unaware of the symptoms. Dehydration occurs when your body doesn’t have enough water. When this occurs, the body will feel fatigued and can’t function properly. Being just a little dehydrated can even alter a person’s mood, energy level and ability to think clearly.
Dehydration can be caused by drinking too many sodas, getting overheated, illness, menstrual cycles and stress. Neglecting to drink enough water can cause you to feel sluggish and foggy because cells shrink from lack of water. The shrinkage of cells is detected by two types of brain sensors, one controlling drinking and the other controlling the excretion of urine by sending a message to the kidneys. When this happens, the antidiuretic hormone vasopressin produces a smaller volume of more concentrated urine.
The amount of water your body requires varies for each individual, even for those who are sedentary. On average, doctors recommend about 15.5 cups of fluids for men and 11.5 cups of fluids a day for women.
