Speech fluency in morning after taking antihistamine at bedtime?

Can anyone give me insight into why I have trouble speaking easily and fluidly the morning after I wake up having taken Zyrtec at bedtime? Interesting that my adult son has the same problem, and it appears to wear off in the afternoon. Does the drug do something to brain speech centers? Does it have something to do with higher cortisol levels in the morning? I've already reduced the dose to the minimum amount to control my allergies, and the other agents (Allegra, Claritin) simply aren't as effective. It does not appear to be an issue with dry mouth or throat. Any ideas on how to prevent this from happening? Thanks for any help you can give.