Seasonal allergic rhinitis, commonly known as hay fever, occurs when the immune system overreacts to airborne particles like pollen, triggering symptoms such as sneezing, congestion, and itchy eyes. Many people find that even if they manage their symptoms during the day, they intensify dramatically at night. While the overall outdoor pollen count often peaks earlier in the day, the worsening of nighttime symptoms is less about the immediate outdoor concentration and more about a combination of meteorological factors, where the pollen ends up, and the body’s own physiological changes.
When Pollen Counts Peak Outdoors
Pollen is typically released from plants during the morning hours, often peaking in concentration between mid-morning and early afternoon as temperature rises and humidity drops. This release is driven by the plant’s reproductive cycle and is helped by the day’s warming air, which creates strong vertical air currents that lift the tiny grains high into the atmosphere. For people living in rural areas, this mid-day timing often corresponds to their highest outdoor exposure.
As the sun sets and the ground cools, the air closest to the surface also cools, which can lead to a phenomenon where the air becomes stable and vertical air movement decreases. Pollen grains that were lofted high during the day begin to descend and settle near the ground again, which can increase localized concentrations at the surface during the evening. In heavily urbanized environments, this settling, combined with the delayed release patterns of certain urban plants, can cause the highest exposure to occur in the evening, making the air near the ground dense with allergens just as people return home.
Factors That Worsen Symptoms While Sleeping
Even if the outdoor count has settled, the primary reasons symptoms worsen at night are directly tied to the body’s internal clock and the bedroom environment. The human body operates on a circadian rhythm, which includes a predictable fluctuation in hormone levels throughout a 24-hour cycle. One significant change is the drop in cortisol, a naturally produced anti-inflammatory hormone.
Cortisol levels are highest in the morning and reach their lowest point around midnight, reducing the body’s natural suppression of inflammation at night. This drop allows inflammatory reactions, like those triggered by pollen, to intensify. Furthermore, histamine, the chemical released by the immune system that causes allergy symptoms, naturally peaks between midnight and 4 a.m. This timing means the body is producing more of the chemical that triggers sneezing and congestion precisely when its natural anti-inflammatory defense is at its lowest point.
Adding to the physiological shift is the concentration of allergens in the sleep environment. Pollen sticks easily to hair, skin, and clothing throughout the day, and these grains are then transferred and accumulate in bedding and on pillows. When a person lies flat to sleep, gravity no longer assists in draining mucus from the sinuses, allowing it to pool and increasing the sensation of congestion and post-nasal drip. This combination of localized indoor allergens and positional congestion, amplified by the body’s nighttime hormonal changes, creates the perfect environment for a severe allergic reaction during sleep.
Practical Steps for Better Sleep
To minimize the compounding effects of nighttime pollen exposure, it is beneficial to establish a barrier between outdoor allergens and the sleep environment.
Recommended Mitigation Strategies
- Shower and wash your hair before bed to remove pollen particles collected during the day. This prevents the transfer of allergens from your body directly onto your pillow and sheets.
- Keep bedroom windows closed, particularly during peak pollen seasons, to prevent outdoor air from contaminating the room. Using an air conditioner to cool the space instead also helps filter the air.
- Install a high-efficiency particulate air (HEPA) filter in the bedroom to continuously capture airborne pollen grains, dust, and pet dander.
- Wash bedding at least once a week in hot water, specifically 130°F (54°C) or higher, to destroy and wash away accumulated allergens like pollen and dust mites.