This is probably obvious to a lot of people, but I thought it still might be useful since I didn't think of it myself for years. Since sometime in college, I've been the kind of person who wakes up groggily at 10 am and then drags myself out of bed. I've always been a late sleeper so I didn't think much about it, but when I stopped taking allergy pills, I fairly quickly became someone who woke up around 8:30 am without much effort (still not early but not crazy-late).
I was aware that allergy pills make you sleepy, but the actual effect when I take them has never been that strong for me, so I never put together that they might still be having a major effect 10+ hours later. I still occasionally take allergy pills in certain cases, and now that I'm looking for it, it's really obvious how they effect my next morning.
For what it's worth, of the two brands that work on my allergies, 6-hour Benadryl and 24-hour Zyrtec seem to have the same effect on my mornings despite claiming different time-periods.
So, if you take allergy pills and find it very difficult to wake up in the morning, consider trying not taking them for a few weeks. There was a reason I was taking them and this was a minor decrease in my quality of life, but getting 2 more hours of time in the morning more than makes up for it.
Ditto, and I'd add that sensitivity to antihistamine side effects varies dramatically from person to person. I spent a long time thinking I had a sleep disorder, and was prescribed modafinil for years, but it turned out to be mainly a combination of zyrtec + depression (plus a few other contributors, but these were the largest non-genetic factors). I'd been on either zyrtec or claritin every day since I was ~8 years old, and they only started causing these kinds of problems in my early 20s. Now even Allegra D or olopatadine cause drowsiness and emotionlessness for me if I take them for more than a day or two at a time. (Flonase helps me but causes bloody noses, montelukast seems to have no side effects but provides minimal benefit for me. Planning on getting surgery later this year to try and improve my breathing.)
While we're considering stuff: if you have persistent seasonal allergies, consider a steroid nasal spray rather than antihistamine pills. Different profile of side effects, and often more effective.
I was actually taking allergy pills because they help with my asthma symptoms, and nasal sprays don't seem to help me. I started taking Singulair / Montelukast about halfway through when I was taking Zyrtec every day, and it seems to be more effective without the sleep-related issues (although the combination was still more effective for my asthma than either alone).
But yeah, for normal allergy symptoms, focusing the medicine on your nose instead of your entire blood stream is also a good idea. Theoretically there's even an antihistamine nose spray now (Astepro) although I haven't tried it.