There are thousands of living organisms which are living in the mouth and some of them produce a fetid smell.
People who do not clean their mouth, tongue and teeth properly have more of these bacterias. Saliva checks the growth of bacteria but during sleep, saliva production almost ceases and bacteria go on multiplying. Thus, many people complain that they have a bad odor from mouth esp. In morning on rising.
Many types of mouthwash are available in the market, all of them contain alcohol. They help for an hour or two and their excessive use produces more bad smell in the mouth than it was before their use.
The best way to stop bad odor from the mouth is maintaining good oral hygiene by brushing teeth properly twice daily along with cleaning of the tongue by a tongue cleaner. Homeopathic remedies can do rest of the work to stop bad breath.