For the past six months, I wake up with dry mouth and throat and saliva after sleeping for four hours. I can usually go back to sleep, but then I keep waking up an hour or so later. I have to drink lots of water to go back to sleep. My eyes and nose are dry too. But I'm fine during the day. Can somebody help me resolve this?

Are you on any medications for depression, allergy, or for sleep? If so, these may be the cause. You can try a humidifier in your room if you live in a dry climate or run the ac/heat at night.