I'm using Zyrtec (Cetirizine Hydrochloride), which among the second-gen allergy pills is more likely to cause drowsiness. My primary indicator is lack of sinus headaches at night and in the morning, there could be some correlation to sleeping through a headache if I'm drowsy because of it, but I also seem to be clear during the morning and day, and before going down this path I was lucky if I could go a month without just being miserable due to a headache. It's probably worth it for me to try another of the second gen options.