I have hives during sleep and sometimes night time for over a month already and can be only control by zyrtec. cannot find anything or pattern that trigger the hives reaction.

Hives: This type of recurrent hives may be very elusive in finding a cause. Although most cases of hives are allergic reactions to something internal or external(like exercise , temperature change), there are cases that have no obvious allergic cause. Nevertheless you may want to consult an allergist for a workup and advice. You're fortunate that zytec is that effective.