We like it or not, teeth will get darker over time. This happens due to staining from foods, drinks or smoking. Tea, coffee and red wine are the most popular drinks which stain the teeth. If you want to prevent or at least postpone this staining process, the best is to drink some water or rinse with some water after each time you have anything which you think will stain your teeth. Just be careful not to drink cold water straight after a hot tea or coffee as it will cause tiny cracks in teeth.
Teeth whitening works by using peroxide products to release oxygen ions to break down the bonds in stain molecules. In other word oxygen ion breaks the stain molecule to two or more part which is not black anymore.
Carbamide peroxide and hydrogen peroxide are used for this process. Carbamide peroxide breaks down to urea and hydrogen peroxide. 10% carbamide peroxide equals to about 3.3% hydrogen peroxide. They both work the same when delivered in the same method and concentration. For night bleaching we usually use carbamide peroxide as it takes time for that to break down to hydrogen peroxide and release oxygen ion over longer time. Hydrogen peroxide releases same amount of oxygen ion quicker which makes that suitable for day bleach.