Sadly the CAP 371 rules assume you can sleep to order and have no life...
so if you have to work at say 1600 local you can do 12.25 hrs duty, ie finish at 04.15 local, but the chances are you woke up at 7 or 8am when the rest of the family did and then did other stuff until going to work. Assuming after ending that duty you get home at 0600 you ethieir have to go to bed and be woken up in a hour or so or stay awake because you are then going to work at 2100 that night... do this a few times with local time changes and you get VERY tired!