Niall,
Unfortunately; you have run into the problem of alarms without "times" issue. At midnight the alarm will go off for any event your scheduled to get an alarm from, due to your PDA being "ON" at midnight, it assumes you are using it.
The same would be true if you left your computer on and Outlook open and running.
Microsoft posted this
webpage, and even recommeded a couple of apps that might help you with your problem.
While this is not an inline fix for your problem, it might give you better control of your system.
