Nasal congestion isn’t caused by too much mucus, despite what it feels like. It’s caused by swollen blood vessels inside your nose. The tissue lining your nasal passages is packed with a dense network of blood vessels, and when those vessels dilate and fill with blood, the tissue swells inward, physically narrowing the space air has to pass through. That swelling is what creates the “stuffed up” sensation.
The Blood Vessels Behind the Swelling
The inside of your nose contains bony ridges called turbinates, with the lowest pair (the inferior turbinates) being the largest. These structures are responsible for most of the warming, humidifying, and filtering your nose does before air reaches your lungs. The turbinates are lined with mucosa that contains a unique type of blood vessel: large venous sinusoids, sometimes called “swell bodies.” These vessels function like erectile tissue. When blood fills them, the mucosal lining thickens substantially. When it drains, the lining thins back down.
Under normal conditions, this system works in a steady rhythm. Your turbinates alternate between swelling and shrinking from one side of your nose to the other in what’s known as the nasal cycle. At any given moment, one nostril carries more airflow than the other, then they switch. You usually don’t notice this. But when something triggers widespread swelling on both sides at once, you feel congested.
What Triggers the Swelling
Several different pathways can push those blood vessels into a swollen state, which is why so many different situations leave you congested.
Infections
When a virus like the common cold infects the cells lining your nasal passages, your immune system responds with inflammation. Immune cells release chemical signals that widen blood vessels and make their walls more permeable, allowing fluid to leak into the surrounding tissue. The result is both swollen turbinates and excess mucus production, a combination that blocks airflow from two directions at once.
Allergies
In an allergic reaction, the sequence starts when an allergen contacts mast cells in the nasal lining that have been primed by previous exposure. Within minutes, those mast cells release a burst of chemical mediators, including histamine and leukotrienes. Leukotrienes are particularly linked to congestion because they cause prolonged blood vessel dilation and tissue swelling. Nerve fibers in the nasal lining also get involved, releasing compounds called neuropeptides (like substance P) that cause plasma to leak from blood vessels into surrounding tissue, adding to the swelling.
Cold or Dry Air
Your nose has to warm and moisten every breath before it reaches your lungs. When the air is cold and dry, this puts extra demand on the nasal lining. The irritation triggers your nasal glands to produce excess mucus to keep the lining moist, and blood flow to the area increases to supply more warmth. That increased blood flow engorges the turbinates, which is why stepping into frigid weather can make your nose feel simultaneously runny and blocked.
Your Nervous System Runs the Show
The autonomic nervous system, the same system that controls your heart rate and digestion without conscious input, directly regulates how congested or open your nose is at any given moment. Two branches compete for control. The parasympathetic branch promotes blood flow to the nasal lining, increases mucus secretion, and generally makes the nose more congested. The sympathetic branch does the opposite: it constricts blood vessels, reduces secretion, and opens up the airway.
At rest, parasympathetic tone dominates, which is why lying down often makes congestion feel worse. When you exercise, sympathetic activity ramps up, constricting nasal blood vessels and opening your airways. This is also why decongestant medications work: they mimic sympathetic nerve signals to shrink swollen blood vessels.
When the balance between these two branches gets disrupted chronically, the result is vasomotor rhinitis, a form of persistent congestion triggered by things like temperature changes, strong odors, stress, or certain medications, even without an infection or allergy present.
Why Congestion Feels Worse at Night
Gravity plays a straightforward role. When you’re upright during the day, blood drains downward from your head, keeping nasal blood vessels from overfilling. When you lie down, blood pools more easily in the vessels of your nasal lining, increasing the swelling. Parasympathetic nervous activity also tends to increase at night, promoting more secretion and vessel dilation. Combine those two factors and it explains why a stuffy nose that felt manageable during the afternoon becomes miserable at bedtime.
How Decongestants Work (and Don’t)
Nasal decongestant sprays containing ingredients like oxymetazoline work by directly stimulating sympathetic receptors on the blood vessels in your nasal lining, causing them to constrict rapidly. The relief is fast and significant. But these sprays carry a well-documented risk: if used for longer than about three days, they can cause rebound congestion, a condition called rhinitis medicamentosa. The nasal tissue becomes dependent on the spray, and when it wears off, the swelling comes back worse than before, creating a cycle that can be difficult to break.
Oral decongestants have their own issues. The FDA conducted a comprehensive review and proposed removing oral phenylephrine, the most common decongestant in over-the-counter cold medications, from shelves after an advisory committee unanimously concluded it doesn’t work at standard doses. The concern is purely about effectiveness, not safety. The nasal spray form of phenylephrine is not affected by this finding. Pseudoephedrine, the other common oral decongestant (sold behind the pharmacy counter), does have evidence of effectiveness because enough of it reaches the nasal blood vessels after being absorbed through the gut.
Saline and Other Non-Drug Approaches
Saline rinses and sprays reduce congestion through a different mechanism entirely. Hypertonic saline, which has a higher salt concentration than your body’s fluids, draws water out of swollen nasal tissue through osmotic pressure. Studies on hypertonic saline show this fluid shift happens rapidly, reaching equilibrium within about three minutes. Normal saline (matching your body’s salt concentration) helps primarily by physically flushing out mucus, allergens, and irritants rather than reducing swelling directly.
Steam and warm compresses work by loosening thickened mucus and mildly increasing local blood flow, which can feel counterintuitive since blood flow contributes to swelling. But the thinning of mucus often provides more noticeable relief than the modest increase in vessel dilation. Elevating your head while sleeping uses gravity to help drain blood from the nasal vessels, addressing one of the core mechanical causes of the congestion itself.