Curtain Rod
Do not have system-wide layovers in the 18-30 hour window that is supposed to be avoided rather than standard practice
This regulation was based on UK Charter Airlines operating Day/Night rotations to Orlando using B757 equipment (so a while ago).
There is nothing for example to stop Airlines operating 18-30hrs Night - Day rotations (where 1 sleep at night on acclimatised local time can occur)
The 18-30hr rest period some 30 years on is very much valid, but it depends on the schedule.