Allergies that worsen after the sun goes down are often referred to as nocturnal rhinitis. While symptoms may be manageable during the day, changes in environment and body function at night can lead to sneezing, congestion, and poor sleep quality. This nightly increase is the result of a complex interplay between the sleeping environment, the body’s physical position, and internal biological rhythms. Understanding these factors illuminates why nighttime becomes a period of peak reactivity for the immune system.
The Bedroom Environment
The immediate surroundings where you sleep often contain the highest concentration of allergens. Bedding, mattresses, and pillows provide an ideal habitat for dust mites, microscopic arachnids whose waste particles are potent allergen triggers. These mites thrive in warm, humid conditions, feeding on dead skin cells shed by humans. An average mattress can harbor millions of dust mites, and movement disturbs their particles, making them airborne and easily inhaled.
Limited airflow in the bedroom, especially when windows are closed, allows airborne particles to concentrate. Pet dander, if pets are allowed in the room, also accumulates heavily in bedding and carpets. Mold spores can be a factor, particularly in areas with high humidity or near windows where condensation occurs. Controlling these factors, such as washing bedding weekly in hot water and using allergen-proof covers, reduces nightly irritant exposure.
Positional and Physiological Changes
Lying down introduces mechanical and physiological changes that intensify nasal symptoms. During the day, gravity helps drain mucus from the nasal passages and sinuses, but when the body is horizontal, drainage becomes less effective. Mucus pools in the upper airways and throat, leading to congestion, postnasal drip, and coughing. This pooling can also increase pressure in the sinus cavities, exacerbating a stuffy nose.
Lying flat also causes a redistribution of blood flow throughout the body. More blood flows to the head and neck area, causing small blood vessels inside the nasal passages to swell. This swelling, known as vasodilation, further narrows the nasal airways already inflamed by allergens. Elevating the head with extra pillows or a wedge can use gravity to aid drainage and reduce blood flow to the head, offering relief.
The Body’s Internal Clock
Beyond environmental and positional factors, the body’s 24-hour biological rhythm, known as the circadian clock, plays a significant part in the nocturnal worsening of allergy symptoms. This internal timing system regulates the release of various hormones and inflammatory mediators, creating predictable daily fluctuations. One such fluctuation involves cortisol, a steroid hormone naturally produced by the body that acts as an anti-inflammatory agent. Cortisol levels naturally peak in the early morning to help the body wake up and then gradually decline throughout the day.
This means that in the early morning hours, typically between 2 a.m. and 4 a.m., the body’s natural anti-inflammatory defense is at its lowest point. This drop reduces the body’s ability to suppress the inflammation caused by allergic triggers. Simultaneously, histamine, the chemical released by mast cells that is responsible for allergy symptoms, often follows a different circadian pattern. Research indicates that histamine levels can be naturally higher during the night and early morning, leading to a peak in allergic reactivity. The combination of higher symptom-causing histamine and lower symptom-suppressing cortisol creates a biological window of vulnerability that makes allergic reactions most severe during the deepest sleep hours.