As you sleep, your body is making repairs and recovering from the hard 24 hours you just survived. It uses all of the nutrients and liquid it can to heal the damage done to your muscle tissue via your workout, as well as trying to reset your central nervous system. This is why your mouth is so dry when you first wake up in the morning. However, it's not just your mouth that's dehydrated, but your entire body is feeling the thirst. If you've ever read articles on how to get started running, you'll find that one of the key tips is to stay hydrated. Most experts will tell you that by the time your mouth feels dry, you're already more dehydrated than you want to be. Dehydration leads to fatigue, not to mention reduced body function. To combat this problem, down a glass of water first thing in the morning. Have a water bottle sitting on your nightstand, and drink it before getting up to brush your teeth, use the bathroom, or prepare for the day. It will help to revitalize your organs and muscles, and will help to boost energy production first thing in the morning.