I have a thick, yellowish-brown discharge in my saliva when I go to sleep. It is only present while I am sleeping, not while I am awake or at any other point throughout the day. I was wondering: Could it be sinus drainage, or what?

D.R.

This is not at all unusual. Unfortunately, the explanation requires that you accept the following discomfiting fact:

MUCUS IS ONE OF YOUR PRECIOUS BODILY FLUIDS.

Your sinus and nasal cavities produce about one liter (that's four cups American) of mucus in 24 hours. Mucus is produced by the thousands of microscopic mucus-secreting glands that are scattered throughout the sinus and nasal cavities. Mucus helps humidify the air you breathe; it also filters impurities from the air. "Impurities" include dust, pollen, microorganisms, smoke, ash and anything else that doesn't belong in your lungs. Under normal circumstances, this mucus/garbage mix drains down the back of your throat and you swallow it.

To rephrase your question, you are actually asking two things: Why is this stuff thick and yellowish brown, and why don't you swallow it at night, like you do during the day?

Sinus infection is a very common cause of discolored mucus. Not all sinus infections are accompanied by "sinus headache," by the way; in some cases, discolored mucus is the only symptom of sinusitis. Most commonly, sinusitis is due to a population explosion of bacteria in the mucus. Hey, mucus is good food (if you are a bacterium). As the bacteria eat, divide, excrete waste products and die, they change the color of the mucus from clear to yellow. (Green and brown are also popular colors.)

Bacterial infection may also increase the viscosity of the mucus, making it more difficult to swallow. During the daytime, the process of swallowing mucus is facilitated by eating and drinking; at night, thick mucus may pool and cling to the back of the throat. It is also possible that your sinuses drain better when you are supine (lying on your back).

Other things besides bacterial infection may discolor mucus. Contrary to popular belief, milk does not "produce phlegm"; milk only turns mucus white, giving it a more dramatic appearance. Similarly, chocolate will turn mucus brown. Chewing tobacco, tobacco smoke and any particulate matter that you inhale at work (or at play) will similarly discolor mucus.

If you do not smoke or chew tobacco, if you are not around secondhand smoke and if you have a clean-air job (i.e., you are not a mechanic, chimney sweep or coal miner), then you could have a sinus infection.