The most common cause of what you have described is dry mouth or what is called mouth breathing. This coating appears when there is not enough saliva at night. This is very common for those with sinus problems, colds (as they can't breath through the nose and do so from the mouth.
You can try using a misting humidifier in your room as this will keep moisture in the air.
Does it cause pain or feel really ruff when you rub it? Thanks
This could be fungal related. You don't mention anything about a body rash or fever, as that could indicate Scarlet fever. You don't mention any open sores.
Without seeing it and if you have no other symptoms, it's either fungal related, which is an overgrowth of the papillae on the tongue or dry mouth. But if you are sure that your mouth is NOT dry, then chances are fungal related. Which you can also try eating some yogurt and allow it to sit on your tongue for a bit before swallowing. Which can occur if your immune system is down or if you are having any mouth infections.
If the yogurt doesn't help over the next couple of days, then I would follow up with your doctor for a proper in person evaluation. As you may need a swabbing of the area and sent to the lab.