The reasons for the recurrence of urticaria at night may be vagus nerve excitation, the relative decrease of glucocorticoid secretion at night, and contact with allergens. 1. Vagus nerve excitation: As sleep at night will lead to vagus nerve excitation, manifested as vasodilatation, blood pressure drop, smooth muscle spasm, etc., which can lead to aggravation of the allergic reaction, resulting in the recurrence of urticaria. 2. Reduced secretion of glucocorticoid hormone: human adrenal secretion of glucocorticoid hormone is an anti-allergic, anti-inflammatory ability of the hormone, its secretion has a circadian rhythm, generally 6-8 am secretion level is the highest, and then gradually decline, the night 0-2 pm secretion level is the lowest, so it is easy to recur at night urticaria. 3. Contact with allergens: urticaria can be caused by contact with allergens, night rest, if there are allergens in the room, such as dust mites, pollen, animal lint and other substances, can also lead to night recurrence. As urticaria is easy to recur, once diagnosed, patients should be standardized treatment under the guidance of the doctor.