Hundreds of salivary glands in the mouth produce, on average, about a quart of liquid a day—enough to fill two medium-size bathtubs a year. But saliva production varies considerably from person to person, by time of day, and under different circumstances. Notably, you produce less saliva when you are sleeping or dehydrated and, typically, as you age.
When you are nervous or frightened, saliva production is reduced. Hot weather, inadequate fluid intake, strenuous exercise, many medications, and some medical conditions can result in decreased saliva production and potentially dry mouth. Smoking or breathing through your mouth dries up saliva.