I fell asleep for three hours without brushing my teeth last night, should i be worried?

Just one time?: If this is not a repeated incident, it is unlikely there is any irreversible damage done on your teeth or gums yet. Flossing is very important, besides brushing. Make sure to spend time brushing with fluoridated tooth paste (2 minutes is what it takes to do a thorough job to brush all teeth) as well as flossing in between teeth to remove any food and the bacteria that are feeding ; breeding on it!