JWP;
In Canada, the Air regs (CARS) provide for "Controlled Rest on the Flight Deck" - the applicable standards are found at CARS 720.23
Part VII - Commercial Air Services - Transport Canada
I have used such techniques on both domestic and long-haul operations when needed by a crew member for whatever reason. About twenty minutes sleep is about all that is required. Anything longer than around 45 minutes is to be avoided because it takes one into REM sleep, (so our training stated), and sleep inertia then becomes a problem.
Any resting crew member (whether in the bunk, the cabin or doing controlled rest on the flight deck), should be wakened at least 30 minutes before top-of-descent.
The key understanding regarding fatigue is that when the brain/body needs sleep it will take it regardless of circumstances or the "requirement" to remain alert. Machinbird describes such phenomenon quite well in the Air India Express accident thread.
The Canadian Air Regs permit a crew member to be on duty for 20 hours providing there is an Augment Pilot, and a SAE-standard bunk available for prone rest. The duty day may be extended to 23 hours in "unforeseen circumstances".
Such duty days as permitted are unsafe in two-pilot transport flying. Both the ATA and the airlines themselves consistently resist recognition of fatigue. One CEO called crew augmentation, "union feather-bedding".
Pilot associations have long been spending their (increasingly-limited) negotiating dollars on realistic and safe duty day contractual (meaning without legal force) limitations. Bus and truck drivers receive greater support from the regulator than airline pilots.