Not quite sure why you sy overtime should not be available for night shifts?
Surely the aim is to increase the staff, and if by offering overtime for nights you can get extra people on nights or relieve the pressure on those frequently doing nights (there are many who have done 16 sets of nights since O date) then that has to be good?
It does of course cause problems for rostering in compliance with SRATCOH, but that's why we employ vast numbers of people in the ORO, isn't it?