If this is truly clear sinus (or nasal) drainage, the excess mucous is likely to be caused by allergy. In that case an over the counter (OTC) decongestant, anti-histamine or a medication that is a combination decongestant/anti-histamine would probably provide good relief. Some of these preparations are labeled for night-time relief and also contain cough (prevention) medication.
You might want to consider any recent changes in your work or home environment that could expose you to allergens or irritants that could be the cause of your symptoms. In that case, avoidance or reduced contact with the substance would be the best approach.
If your symptoms are not relieved by the OTC med(s), within 1-2 weeks, you should see your family doctor.
Good luck
I'm still working out a similar problem myself. What works for one doesn't always work for another so some people might tell you to try something else.
Slightly elevating your pillow can help you sleep - this stuff drains down your throat and can tickle or make your throat dry and therefore make you cough and wake you up.
A sinus rinse really helps alot. I just got my Mom started on one as her allergies are so bad and she's now addicted. It's helpful and harmless. It's a saline rinse that you do in your nose.
Drinking alot of water can help the fluids as well.
I'm currently seeing an allergist for this problem too - the post nasal drip seems to be making me very sick to my stomach - but the above things have helped me some. I'll keep you posted if the allergist comes up with more to help - he gave me a prescription for a sinus spray that helps with post nasal drip - I haven't filled it yet (I need to!) but perhaps that'll help too.
Good luck!