It's a good question although not Cathay specific.

I have done longhaul overnight flights regularly for the past 4 years and have worked out my sleeping routine fairly well. It's all fairly commonsense:

• Choose flights that have a clear 6-8 hours in the air at a time when you will be really tired (i.e. from midnight local time). Don't include the first 30 minutes after takeoff or the last 30 minutes on descent. I love flights that depart just before midnight.
• Have a busy day before and don't drink too much caffeine during the day.
• I avoid melatonin, I don't like the quality of the sleep I get and if you can't sleep it leaves you in this weird drowsy state. I've read antihistimines are better but haven't tried them.
• A little alcohol is okay but not too much, it makes me uncomfortably warm, also any hangover would be multiplied.
• Hydrate - but also go to the bathroom before you sleep so your bladder won't wake you in the night.
• If the meal would cut into your sleeping time, just eat before the flight in the lounge and skip the meal. The food in the lounge is better anyway.
• Bring pyjamas. I hate sleeping in clothes, for some reason my legs always sweat.
• If it is a short flight, I change into my PJs before takeoff, that way I can put my bed flat and go to sleep before the seatbelt sign goes off. My PJs are enougn like clothes that if we aborted takeoff and had to return to the gate I could swing it.
• Practice sleeping with a mask and earplugs. You just have to get used to it.

Even with all this, sometimes you just can't sleep. It's hard to drift off even in mild turbulence. Sometimes you're just not tired. If you lie there for a while and just can't sleep, don't panic, just stop trying for half an hour, read a book or something, and then give it another go.