Your saliva (spit) provides a protective layer in your mouth to prevent bacteria from growing on the surface.

However, when you sleep at night, you produce less saliva so you don't drool or choke on your spit. Less saliva means the bacteria can spread over the surfaces of your mouth. The foul smell of morning breath is the waste emitted by the bacteria as they snack on any food remaining in your mouth.

If you leave your mouth open or breathe through your mouth at night, your mouth dries out faster and allows the bacteria to spread quicker.

Brushing your teeth and tongue helps get rid of the bacteria producing the foul smells.