Sorry to be the bearer of bad news ,but 20 mg is way to much ,your on the threshold of it being bad for sleep ,you should really try tapering your dose down . I have seen different responses from different brands .you might want to try buying the 5 mg of different brands and try cutting back on it slowly ,see which is more effective for you at what dose ,if you take it every night at that dose it's probably hurting your sleep more than helping .

Ps. Almost every antihistamine and antipsychotic can cause retention on the short term when initiating, unfortunately long term it can cause permanent retention issues , I am like a a "male dog always got some to mark his territory" i had bad allergies and anti histamines caused me to not fully empty , until it built up and hit a trigger point where my bladder says "everybody out" Overflow incontinence ,one wetting is an extinction level event for most diapers ,so I have to catheterize every 4 hours .almost all psychoactive drugs are derived from antihistamines be they antidepressants ,antipsychotics, etc etc