Conditional AND is most definitely possible with the apex.
You do so like this
Set OFF
(Low priority alarms)
If Time 21:00 to 05:30 Then OFF
(High priority alarms you want alerted for even at night)
Creates the logic of (Low priority AND NOT night) OR high priority.
The kicker here is that in order to do thus, all of your alarms that you want suppressed during night have to be done in code form in the email and set to "never" in the input setup