From Quora:
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Short Answer:
While most dental health professionals advocate at least brushing twice a day and flossing daily, the single-most beneficial time to brush and floss is between your last meal of the day and the time you go to bed, avoiding between those times all food and drink other than plain, natural, unflavored, non-carbonated water.
General Explanation:
When you go to sleep, so do the glands that make saliva and that means whatever is on your teeth as you go to sleep is going to stay there all night and wreak havoc. If your mouth is clean and bacteria-free, there is nothing to cause damage.
Detailed Support:
To keep your teeth and gums as healthy as possible, it is important to keep them as clean as possible and to eliminate as much as possible the factors associated with poor oral health. Obviously, keeping your mouth clean means you should brush and floss regularly. But there is more to it than that. The majority of tooth decay begins with damage to the tooth surface caused by the tooth being exposed to acid. The pH scale is a standard way of measuring how acidic something is. A neutral pH is approximately 7.0, with a lower number indicating a stronger acid and a higher number indicating a weaker acid. As a point of reference, the pH of water is 7.0, and the pH of battery acid may be less than 1.0. Clearly, a reasonable person of at least average intelligence would not consume battery acid. That said, some of the foods we do eat are quite acidic—examples include lemons and limes (pH ranging from 1.8 to 2.4), orange juice (pH about 3.0), not to mention sports drinks and energy drinks (most have a pH ranging from 2.5-3.2). Specific to teeth, a pH of 5.5 or below is considered harmful to a tooth surface.
An acidic environment allows to flourish the bacteria that cause problems with oral health. Tooth decay, bone loss, and periodontal disease develop because of bacteria and the byproducts of those bacteria. Bacteria are living organisms and they eat and they leave behind waste products. Specifically, the acids that cause problems in the mouth are primarily the waste products of two types of bacteria, Streptococci Mutans and Lactobacilli. Typically, the decay process is initiated by S. Mutans and furthered by Lactobacilli. Their waste products combine to form Lactic Acid (pH 3.5–4.0).
Why all this talk about pH? Because removing the acids and creating a more basic environment in our mouth reduces the amount of breakdown of tooth structure and periodontal structures which leads to enhanced oral health. As mentioned above, water has a pH of about 7.0. Saliva is pretty much water, but the pH of saliva in a healthy person can range from 4.5–8.0 and can be a function of flow rate (the higher the flow rate, the higher the pH, meaning the more basic). So essentially, a strong flow of saliva is good for our oral health. That’s why I mentioned earlier that our saliva-producing glands shut down at night, meaning the flow rate is very low—one reason for making sure our mouths are as clean as possible before going to sleep.
Now, what about brushing after meals? When we eat a meal, we create in our mouths an acidic environment, which can micro-etch the surfaces of our teeth (which segues into some of the detriments of frequent snacking, but we can save that for another discussion). Most toothpastes contain small abrasives. Brushing immediately after a meal certainly removes the excess food particles and leaves us with a minty-fresh feeling in our mouth. But the combination of etched tooth surfaces and abrasive toothpaste can actually cause an increased rate of tooth surface erosion. Studies advocate waiting about 30–40 minutes after eating before brushing, suggesting that such a delay allows for the pH of our mouths to return to neutral before introducing abrasive toothpaste. I suggest rinsing vigorously with water immediately after each meal. This removes excess food particles, washes away a good amount of the acids created while eating, helps reduce your mouth’s population of bacteria and their waste products, and hastens the return of a neutral pH to our oral cavity.
Conclusion:
Brush twice a day and floss daily. Brush twice and floss daily. I say again, brush twice and floss daily. Cleaning your mouth in the morning certainly has benefits and a dentist who coaches you to do so is giving sage advice. For at least the reasons discussed above, a thorough cleaning of the entire mouth before going to sleep is highly beneficial. I strongly suggest doing both—brush in the morning, brush and floss at bedtime. Cleaning your mouth only once a day is a compromise from the ideal. That said, my professional opinion is that if someone was absolutely determined to choose one and only one time per day to clean one’s mouth, bedtime would be the most beneficial.