Mood dip in the spring? Can't stay asleep? Brain fog zapping your mojo? One of the first things I'll recommend to patients who report a new onset of symptoms December-March is to check the pollen counts. There's plenty of evidence of allergies causing way more than just itchy eyes and a sore throat.
Allergies, of course, occur when your immune system detects an allergen--a substance which causes it to overreact--leading to sneezing, itching, or hives. Immune system responses are called inflammation, and the whole-body inflammatory reaction happens because chemical signals called cytokines are released by white blood cells. These signals serve to let other parts of your body know that something is wrong.
The cause of allergy-associated depression is not clear, but it may be that these signals are trying to tell your brain that the body needs to rest. So as a result, you feel tired, with low motivation, and don't really want to do much of anything. This sure feels a lot like depression.
In other patients, these signals lead to anxiety, the uncomfortable feeling that something awful will happen. It's no accident that hydroxyzine, an FDA approved antihistamine, is also approved for the treatment of anxiety. The two are linked.
Speaking of histamine, this compound is why your sleep is disrupted during allergy season. A type of immune system cell called a Mast Cell releases histamine when it comes across an allergen. Histamine, in addition to causing hives and itching, promotes wakefulness. You require a certain amount of histamine in your system in order to be awake. Thus, antihistamine medications make you sleepy (even the "nonsedating" ones can cause some somnolence). And with lousy sleep, it's hard to function the next day--thinking gets cloudy, and we don't handle stress as well.
This may contribute to the concerning link between allergies and suicide. Suicides peak in the spring. If you're already a mess due to depression or mood instability, and now you can't sleep or think straight, this can lead to a pretty awful outcome.
So, to summarize, if you recognize that you're in a rough spot with moods, and you can't figure out another cause, take a look at the local allergy report. If you notice that the pollens are high, consider an over the counter antihistamine (provided, of course, that's ok with your doctor). You may find your mood and overall outlook a lot better pretty quickly.
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