You should brush, 20-30 minutes after eating on the night (just before you sleep) and, 10-20 minutes before eating your breakfast. Midday brushing, is not essential for most albeit, it's highly dependent on your dietary habits. The same rule applies: 20-30 minutes after eating or 10-20 minutes before eating. The time-rule is influenced/is-in-place to compensate for the time it takes for the normalization of the oral PH after food consumption ("after a meal"), and the latter being to ensure that the introduction of food stuffs and liquids will not diminish the concentration of the essential fluoride, contained within toothpaste, on the tooth surface ("before a meal").
Not brushing after you eat, before you sleep on the nighttime, can be likened to having oral excrement...
Whenever you eat/have a meal, our oral microbiota ingest and metabolize the food particle left over, essentially feasting on whatever we consume. Like every other living organism, these oral bacteria excrete as they reap. However, this secondary excrement is highly acidic and can have some unwanted effects on your tooth enamel: gradually dissolving it and henceforth leading to the occurrence of tooth decay (cavities).
When you decide to "hit the hay" without brushing, considering you have eaten since, plaque may start to harden and hence, calcify on the surface of your teeth. It's important to know that, once calcification of the plaque on the tooth surface has began to occur, we (yourself), cannot remove it with a toothbrush nor with any amount of floss; it can only be removed by a dental hygienist via a routine cleaning using specialist equipment.
It doesn't just stop there! As plaque begins to build up, your immune system treats it as if it's an infection. Your immune system doing what it does best, triggers the release of in situ Prostaglandins, to trigger an immune response via lipid signalling. As opposed to a "positive" immune response, the healthy tissue that adhere the gums to the teeth to the gum surface are attacked and harmed (degenerated) causing it form periodontal pockets and cause gum recession. Periodontal pockets, can predispose plaque formation and hence, foreign bacteria colonisation below the gum line - along the tooth root. This healthy tissue is known as the, attached Gingivae and it doesn't just permit a snug fit, it helps to keep to teeth in place!
Don't get me wrong, inflammation is good thing! Albeit, a long-term presence of inflammation can predispose you to a worrying amount of tissue damage. For example, take the common flu: short-term inflammation is essential for cell signalling, to help route immune factors such as phagocytes, to the site of the harmful bacteria and hence, to ingest and destroy the foreign bacteria -- known as Phagocytosis; helping to make you "healthier" again. However, long-term inflammation causes unwanted bodily "wear and tear", that has been found to correlate with some very serious diseases and impairments such as, Heart disease, Dementia and secondary Hearing loss.
Yes - non of this will happen over the course of one night. But, if it becomes habitual (not brushing before you sleep -- or flossing), the probability of it happening will steeply increase!
Considering that, this small precaution will help save you a heap of money and pain in the future, is it not worth it? Evidence shows that it may even extend your life!
The amount of saliva produced is greatest when we are awake (during the day), and diminishes greatly during natural sleep and this in fact reduces the ability of our saliva to natural control oral PH and partially digest food particles in our oral cavity. The protein profile (and hence, enzymatic activity) of our salivary rate is not the same during rest as that, when we are eating or when sleeping - less salivary flow. Salivary secretion during wakefulness is, in part, associated with oromotor activity involving the masseter muscles. Rhythmic masticatory muscle activity and swallowing are non-disruptive events that occur during normal sleep. It's hypothesised that the natural lubrication that saliva permits is most-necessary during sleep to protect tissue integrity and health of oroesophageal structures.
Mouth breathers, during sleep need to pay particular attention to oral hygiene as the prominence of "dry mouth" is much more evident - this is impirical due to reasons stated above.