Hay fever does more than make you sneeze. Blocked sinuses and a congested nose can disturb sleep, leaving you tired and unable to concentrate during the day, a pattern sometimes called brain fog. Constant nasal inflammation and the body's immune response can also contribute to a general feeling of fatigue, and pressure built up in the sinuses behind the cheeks and forehead can produce a dull headache.
Some older antihistamines cause drowsiness, which can add to tiredness. If this is a problem, a pharmacist can suggest a non-drowsy alternative. Treating the underlying congestion with a steroid nasal spray often improves sleep and reduces both fatigue and sinus headaches over time.
Most hay fever headaches are mild and improve once symptoms are controlled. However, you should see a doctor if a headache is severe, sudden, comes with a high fever, neck stiffness, vision changes, or facial swelling, or if it does not settle with allergy treatment, as these may point to a sinus infection or another cause that needs assessment.