I wake up every morning with a mild throught ache that goes away. what can it be?

Arthritis: If the pain you are experiencing is in a joint, then chances are the diagnosis is that of osteoarthritis. Classically, the discomfort is worse upon awakening and improves as one 'limbers up'. This is a benign process. Discuss treatment options with your doctor if it becomes more bothersome.

Mouth breathing: If you mean a throat ache - you are most likely a moutjh breather which dries the throat and it takes about an hour for the mucosa to moisten again.