I drink water before sleeping. Yet every morning I wake up with very dry mouth. I don't sleep with my mouth open. Not diabetic. What cud be the reason?

Dry mouth. You may be sleeping with your mouth open even if you don't know it and for at least part of the time. Some medications cause dry mouth. When was you last dental and medical check-up? Dry mouth promotes cavities. Make sure you properly brush and floss before you go to sleep.
Stress, see PCP. According to latest research, stress can lead to bruxism and dry mouth. Ask your doctor about sleep hygiene instructions.