In this case, a white milky tongue refers to the entire tongue being coated. If the tongue only has white spots or is only white in some places, there is likely a different diagnosis. If you wake up in the morning with a white milky coating on your tongue that was not there the night before, have a metallic taste in their mouth and bad breath that brushing will not cure, then the causes are fairly simple. This white coating is harmless though unpleasant, and it can be prevented with good oral hygiene.
When a tongue takes on a milky white coating overnight, the reason is simple. The papillae, which are the small bumps that have taste buds between them, become inflamed overnight and swell. As they grow larger, the papillae may catch dead skin cells, bacteria and debris. This is even truer if you breathe through your mouth while you are asleep. These debris lead to the symptoms of a metallic taste and bad breath, as well as the milky white coating on the tongue. With proper brushing and the use of mouthwash, the coating can often be easily removed.
Preventing a white, milky tongue is fairly simple. Brush your teeth regularly so that your mouth is clean. Try to cut back on smoking or quit altogether. Drink plenty of water and not other beverages like tea or coffee that have sugar and caffeine. Lastly, increase your fiber intake. Crunchy foods like apples, popcorn, carrots, celery and broccoli tend to scrape debris off the tongue when they are eaten and can help prevent the development of a white milky layer through the night.