Here's what I learned from my doctor when I had the very same problem...

Coughing is a natural reflex produced when an allergy or other conditions
cause phlegm and other substances to become blocked in the Eustachian tube
and sinus passages thereby draining into the upper lung tissue, particularly
when lying in a prone position.

The coughing at night, even before you actually go to bed, is due to your
body relaxing enough to allow the natural coughing reflex to manifest
itself.  You may not be aware of yourself relaxing enough even prior to your
actually going to bed, but if bed time is usually sometime near the hour of
10pm, then your body begins to relax by habit around that time frame for the
coughing to be produced prior to your going to sleep.

The best thing to do to help alleviate this problem is to continue taking
your allergy medication as prescribed by your doctor, drink plenty of WATER
during the day, and clear your nasal passages as often as you feel necessary
prior to bedtime.  Consumption of large amounts of dairy products, such as
milk and yogurt, as well as smoking will exacerbate the problem; try to
avoid these substances until you are fully recovered.  Likewise, prop
yourself up from about the mid-chest area when you sleep at night, instead
of sleeping in a prone position.  This may help keep the drainage from your
sinuses to your esophagus, rather than to your lungs, and a better ability
to relieve your body of the substances causing your problem.  You can take
an expectorant cough syrup prior to going to bed, and during the day, to
help the phlegm to be expelled from your body.  Please, as always, speak
with your doctor about any drug interactions prior to self medication of
this kind.

You might want to try some Guaifenesin to help loosen and bring up the
phlegm.  Usually works for me when I have chest congestion with cough.  As
for the night time occurrance...happens with me too, not sure why.
~Patty~
snip..

I find that the best thing to do, by far, is to irrigate my sinuses
frequently to flush away those secretions before they even have a chance
to drip down into my larynx and cause the cough.

In fact, I have a mild sinus infection right now that is causing just
such a post nasal drip.  So what I do is irrigate right at bedtime.  And
if I wake up in the middle of the night to go to the bathroom, I will
irrigate again right then.

I find that an expectorant works well to thin the remaining secretions.

<snip>

<snip>

Yellowish snot is a sign of a possible sinus infection.  (Been there,
done that.)

Use saltwater spray to irrigate the inside of your nose.  (Ayr brand
is best.)  Use lots of spray so that you almost drown.

If the problem persists then see your doctor.  Keep using the
saltwater spray while taking the prescribed antibiotics.  Note that
some sinus infections are resistant to up to 5 different antibiotics,
so the first medicine may not work.

SNIP

Several possible reasons.

Pollen and other allergens have  seasons and vary with time of day.

If you go to work and work in an air conditioned building then the air
may well be filtered ( operating theatre staff are well aware of this
especially orthopaedic theatres. Hence you are better by day

You might be reacting to  specific local problem. ( Famously one
problem in  the area of a dockyard only happened when a particular
cargo was unloaded and the wind was in the right direction.The cargo
was also seasonal to make it worse.)

Some plants release pollen at night ( wind pollinators) .

atmospheric conditions also play a part.

As others have said yellow sputum can mean infection but it can also
occur in asthma without infection.

N

Starts at bedtime and clears up a couple hours after waking?

Dust mites in your bedding.  Everyone's got them, you're sensitive to them.

Bleach your sheets/pillow cases, then go to the department store and buy a
"mite-proof" bed bag and pillowcases.

CC

Years ago i was suffering from persistent coughing for 8 months until
an Irish doctor asked me to inhale liquid menthod.

FP
======================

Try inhaling liquid menthod.  It worked for me years ago after
suffering from persistent coughing for 8 months.

FP
=====

In misc.survivalism Andrew <analystresearch2002@yahoo.com> wrote:

Well, you certainly came to the right place for reliable advice! Had you
consulted a "medical" text, you would have been filled with all sorts of
misinformation.  Them "doctors" don't know nothing.

If you want to SURVIVE this, then ask a survivalist.

Ok being a sinusitis sufferer since I was born I can tell you it sounds
like your infected with bronchitis brought on by sinusitis.
First I suggest getting to a doctor and having him check your lungs.
If he says they are OK then try the following

Bed sheets - Bleach them change them bleach them again daily for two
weeks. Lemon juice might help the load too.

Buy a vaporizer - use it to help moisten the lungs the air your
breathing could be too dry. Use it as need.

Lemon tea - drink it hot with a teaspoon of vinegar and a teaspoon of
honey added to the mix stir vigorously. (Helps the sinuses.)

Change all air and heat filters put in a heppa filter if you can.
Change these filters monthly (due to the severity of the coughing)

If you have cats give kitty its own home at the other end of the house.
Cat dandruff is a main cause of the problems around my area. They get
under your house and have kittens.

If the Mrs. likes bringing in flowers remove them immediately from the
house. Biggest source of irritants of the sinuses is pollen

Hope it helps.

A small amount of salt placed under the tounge before going to bed.

I don't know if this answer has been given, but I will cough (madly) at
night if the temperature in the room falls below a certain degree; and I
don't know if this might relate to barimetric pressure as well.  The
temperature doesn't have to be  cold for me to start coughing, but just cool
enough to instigate the coughing.

If I am in a hotel room where the a/c hasn't been cleaned well, I will cough
when it comes on.  Watch to see if perhaps the a/c or furnace turning on
initiates your coughing.

Good luck,
Dee