Most people will have been told at some point in their lives that it's best for tooth brushing to occur right before going to bed. Of course, it's easy to forget to do so, and some people assume that they can just brush their teeth at other times of day without having any impact on their oral health. Unfortunately, when you brush your teeth is just as important as actually doing it, and there's more than one reason why you need to take care of things just before you hit the hay.
The mouth at rest
The first thing you need to understand is that, just like the rest of you, your mouth is more active during the day than during the night. That's partly because of how much you actually use it. When you eat, drink, or even talk, you're moving your tongue around and helping create more saliva. That might not sound important, but it actually disturbs any bacteria that tries to build up along the surface of your teeth. Saliva is particularly important since it contains plenty of compounds that help break down bacteria and strengthen your teeth.
When you go to sleep, your tongue, for the most part, is still, and you start producing less saliva since your metabolism will be slowed down.
Bacteria at play
Without saliva to work against it or the tongue to disturb it, bacteria is able to thrive much more easily when you are asleep. Of course, this would be the case regardless of whether you brushed before bed or not. However, failing to brush before bed means there will be more bacteria in your mouth on top of better conditions for it to develop.
You see, bacteria feasts on the food and drink that we consume. Just as it needs to take in nourishment, it also needs to excrete waste. That waste is extremely acidic; it breaks down enamel and leave your teeth vulnerable to decay. If you fail to brush before bed, you'll be leaving bacteria undisturbed in your mouth, plus plenty of substances for it to consume and excrete, and this will all be going on when you mouth is least able to put up a strong defence.
Even worse, the plaque formed by bacteria is given a chance harden and calcify if you don't brush your teeth before bed. When that happens, it cannot be removed through brushing or flossing – you'll need to see a dentist or oral hygienist instead.