lets assume you work 6PM to 6AM Mon, Tue, Wed. Sleep from 9 to 5. Do you alter your wake/sleep on your days off?
If not, can you work out at 6PM on Thurs, Fri, Sat, Sun. If you do shift over - why not work out at 8AM Th,Fr,Sa,Su & Mon.

This might work better than trying to get it in on your 12 hrs off.