Sleep, saliva and bad breath.
Excerpt taken from Gut: The Inside Story of Our Body’s Most Underrated Organ by Giulia Anders:
Our saliva contains bactericidal substances in small concentrations.] Our spit is not supposed to disinfect us completely. In fact, we actually need a core team of good little creatures in our mouths. Benign bacteria in the mouth are not totally wiped out by our disinfectant saliva, since they take up space – space that could otherwise be populated by more dangerous germs.
…The fact that we produce so little saliva at night explains why many people have bad breath or a sore throat in the morning. Eight hours of scarce salivation mean one thing for the microbes in our mouth: party time! Brazen bacteria are no longer kept in check and the mucous membranes in our mouth and throat miss their sprinkler system.
That is why brushing your teeth before you go to bed at night and after you get up in the morning is such a clever idea. Brushing at bedtime reduces the number of bacteria in your mouth, leaving fewer partygoers for the all-night bash. Brushing in the morning is like cleaning up after the party the night before. Luckily, our salivary glands wake up at the same time we do in the morning, and start production straight away. Munching on our first piece of toast or performing our morning dental-hygiene duties adds extra stimulation for salivation, and this washes away the nocturnal microbes or transports them down into our stomach, where our gastric juices finally finish them off.
http://www.amazon.com/Gut-Inside-Story-Bodys-Underrated/dp/1771641495/ref=sr_1_1?ie=UTF8&qid=1447288200&sr=8-1&keywords=gut