The nitrite produced by a meal of overnight vegetables is not enough to cause cancer, but you should pay attention to the way it is stored. The real safety risk of overnight meals is the harm of microbial contamination during food storage.
It is recommended that vegetables be consumed within 24 hours to avoid nutrient loss and bacterial growth. Meat and poultry can be refrigerated for 1-2 days; seafood can be eaten on the same day as much as possible; rice and pasta can be stored in the refrigerator for 1/2 days. Pay attention to reheating before eating.