Antihistamines, often used for relief of allergies, are sometimes prescribed to help people sleep. Examples of antihistamines are diphenhydramine hydrochloride (such as Benadryl) and doxylamine (such as Vicks Nyquil). Short-term improvements in insomnia have been noted with antihistamine treatment, but side effects may include daytime sleepiness, dizziness, mental sluggishness, and rapid heart rate.¹