If you sleep well but for some reason always feel tired, the reason could be as simple as not enough water. The rule of thumb is to have at least two glasses of water every hour. What’s making you feeling tired may be the fact that your heart is working harder to pump blood to compensate for the loss of volume of blood due to dehydration. Also, your muscles lack water, so performing any physical tasks can feel like a lot of work.
Sunken eyes or cheeks
When the skin under the eyes looks dark, sunken, or hollow, you should probably drink some water. Dehydration is a very common cause for sunken eyes. Fluid retention under the eyes may lead to the extra “baggage” and for the skin to become more transparent, letting blood vessels and dark tissue underneath show.
Next time you take a few too many seconds to answer a simple question or look at a picture without understanding what you’re seeing, drink some water and try again. A study of the effects of dehydration on cognitive performance and mood among young men showed that even mild dehydration can lead to confusion, decreased vigilance, and poor working memory.