Of course, this could be anything, from a rapid onset of arthritic pain, to bursitis, to infection. However there are a few clues.
First, night pain that awakens you from sleep; second the knee hurts when you bend it, and gets stiff if you sit with it bent; finally, buckling when you stand. These three combined, indicate a mechanical problem such as a torn cartilage possibly combined with a fluid collection, known as an effusion, in the knee
You mentioned no trauma, but didn't say whether you are active or not. You didn't mention redness, or increased warmth, so i will assume they are not present.
My first choice is probably what is known as a degenerative torn meniscus. These occur not from trauma, but rather from wear and tear over time. In many times, these are present but cause no problems. Occasionally, they get irritated and present with the sort of story you've described.
Unlike traumatic torn cartilage, these tears do not always require arthrocopic surgery. You can treat them with rest, ice (not hot) and anti-inflammatories. You mentioned you are taking Advil. I suggest taking it regularly for 10 days, not just when you have pain. Sometimes a neoprene sleeve is helpful.
If the pain persists, then I suggest a visit to the doctor. At that time a thorough exam can be done along with xrays and an MRI if needed. If it is a degenerative tear, sometimes a cortisone injection is helpful. If all else fails, then arthroscopic surgery is an option.
Thanks so much for the information. It was very helpful.