Nicholas 49 has opened a can of worms with this one.
From my perspective, having flown LHR-HKG many times with my previous company, I would try and get some sleep before the one and a half hour drive to LHR and reporting. (Note, it took about 30 minutes to get from the staff car park to the Queens building then). We would depart at about 8pm, but eould get about 5 hours rest in flight as there were four crew on the flight. Landing at 4pm local time HKG wasn't an issue. The return after 48 hours rest was benign as well. Departing at midnight HKG time, about 4pm body time and getting 6 hours in flight rest was OK. It was a good long haul schedule, the best in out network and generally, the pilots didn't find it hard. There were far worse schedules inclusing the ones that had you operating 2 crew for 9 hours on an authorised variation to the FTLs and returning on a departure at midnight body clock time for an eight hour flight.
Whatever the schedule and no matter how hard you try, you just can't get enough quality rest outside you circadian night time. A couple of hours at best, in daylight, with kids in the house, with traffic/aircraft noise, cleaners vacuuming the hallway outside your hotel room and other distractions - your rest during your circadian day is not going to be effective because it's extremely unlikely you will go into the REM stage which is essential for quality rest.
Another thing to note is that although the effect of one flight can lead to acute fatigue which although can be a risk is often overcome by the increase in adrenaline at the critical stages of the flight, the result of a long haul lifestyle is that the acute fatigue is usually present on top of a systematic and debilitating chronic fatigue. This is when it becomes dangerous. Consider doing up to six UK to East coast of the US flights in a month (which I have done). The return flight means you lose one complete nights sleep each pattern, six times a month. Now imagine doing this for months on end and you start to get the picture.
For non pilots to start postulating that long haul pilots are being irresponsible by not resting properly before reporting, you don't know what you are talking about. Perhaps you should be more concerned that in April 2012, the scientificly proven CAP 371 flight time limitations (which in most pilot's opinion are about the maximum limits of safe operation) are going to be replaced with the EASA FTLs which will allow pilots to work for much longer. Unfortuantely, these limits have been pushed by pressure from the industry and are not backed up by any scientific study or evidence. Compare this with the US Fedral Aviation Authority who are currently introducing more restrictive flihgt time limitations after a fatal crash two years ago caused by pilot fatigue.
The press loves to pillory the occaisional pilot who has been caught reporting under the effects of alchohol, perhaps we should take note of the British Airline Pilot's comparison of a fatigued pilot having had only a couple of hours effective rest in the last 24 being intoxicated to the level of having just drunk four pints of beer. I would say from my experience that is pretty accurate -and yet this is perfectly legal.
And to answer your original question Nciholas 49, just imagine that you boss has asked you to work from midnignt to 8am instead of nine to five. How do you think you will be forming at the most critical stage at 8am?
A before some @rse comes on and says "If you can't stand the heat, get out of the kitchen'', I already have. I got out of long haul and into short haul six years ago. I feel much better.