One explanation has to do with gravity and the way that we tend to position our bodies at night. Many people think that congestion and a feeling of stuffiness is caused by excess mucous blocking the nasal passageways. While this may contribute to a feeling of stuffiness the real culprit in congestion is engorged and/or inflamed blood vessels inside the nasal passageways. This is a very simplified explanation, however, as congestion has many causes (more on some of those later).
When we lay down our blood pressure changes, and we may experience increased blood flow to the upper part of our body including our head and nasal passageways. This increased blood flow can make the vessels inside our nose and nasal passageways even more inflamed. Increased blood flow also commonly causes congestion in pregnant woman.
A lying down position also makes it more difficult to clear mucous from our nose and sinus cavities.
For example, while we are standing up during the day mucous is constantly running from our nose and sinuses into the backs of our throat and being swallowed. Most of the time we don’t even notice this. At night, however, this mucous may pool or back up. Many people notice that their congestion starts to improve an hour or two after they get up in the morning That’s gravity doing it’s work.
Even if we aren’t lying down in bed ready to sleep we tend to start lounging in more relaxed and more supine position a couple of hours before bedtime. This is why you might tend to notice your congestion getting worse not long after the sun goes down.
This also explains why people with conditions like a cold virus or allergies might feel their congestion increase at night. But what if you’re not sick and don’t experience any symptoms during the day? Well, studies have shown that even without the presence of daytime congestion nasal resistance (the ability to breathe) is impaired when we are lying down. Also, another culprit you might not suspect could be to blame: acid reflux (heartburn).
Fairly common symptoms of acid reflux or gastroesophageal reflux disease (GERD) include sore throat, coughing, post nasal drip, wheezing, and hoarseness. These symptoms tend to be worse when you wake up in the morning. Many of these symptoms occur when individuals with an impaired esophageal sphincter (valve) and a stomach full of acid lay down at night. The acid in the stomach can migrate up the esophagus and irritate the back of the throat, which also happens to be connected to the nasal passageways.
The really interesting thing is that some medical professionals now believe that there is a definite link between GERD, chronic sinusitis and even nasal congestion.