How come i have bad breath in the morning, even though i brushed before going to bed?

Mouth breathing?: Maybe due to mouth breathing and therefor dry mouth. Saliva keeps odor causing bacteria to a minimum. Consider treating nasal congestion and try saliva substitutes like biotine mouth spray or "rain" by xlear. Gastric reflux couldalso be in the mix.

Sinus drainage: Usually, fluid accumulate on the sinus floor (especially if you have pneumotization of sinus (floor drop down at the missing tooth site in the posterior teeth area) will drain through from the nasal pharynx to the oral pharynx especially when you are lying down.

Bacteria Nightly: Remember, when growing bacteria you need 6 things, and only 6 things, our mouth have all five all the time, and all six when asleep. 1. Darkness 2. Bacteria 3. Wetness 4. Food source 5. Body temperature 6. Left undisturbed it is best to detail clean your teeth before going to bed, but bacteria will still grow, it is impossible to stop the process, all we can do is manage the growth, via oral care.