Xperia Z3 :: Alarm Not Working On Phone

Updated Z3+ to Marshmallow 5.0.2 and subsequently discovered (on the first day back to work) that alarms (native clock) no longer work after the phone is left overnight.
 
Have checked notifications, priority status, display settings etc. Have force closed, cleared cache and re-started. No success.
 
If I test the alarm during the daytime (with normal phone usage) it works fine, the problem only occurs when the phone is left idle for extended periods.
 
Have spent hours on forums looking for solutions, with some references to Doze causing issues, although seemingly not with native clocks and as far as I can tell, not with Xperia devices.