Sleeping aids for long flights [duplicate]

I don't have a definitive answer, and it might simply be anecdotal.

I've done this for many flights from Montreal to Europe (Paris, Rome, London) where you leave in the evening (and usually leave after "normal" diner time).

1. Do not eat before or during the flight. This will put your body in sleep mode.
2. Do not drink coffee or tea (or any caffeinated beverage). Stick to water.
3. Close all your electronic gadgets (even music players) and do not use the on-board entertainment system
4. Get some ear-plugs or noise cancellation headphones.
5. Get some eyes covers or shield your eyes from lights; I use a hoodie because eyes covers are too warm.
6. Try not to be disturbed, tell the flight attendant and seat neighbors that you do not wish to be disturbed (you can even swap places after take-off to make that easier.
7. Dress loosely and comfortably.

I don't think I ever took sleeping aid (natural or chemical), so I cannot help you for that.

Lately, I found that I sleep a lot better when I am cold. For my next trip in April, I will skip on the blanket.

I would not spend a lot of effort trying to get to sleep when I know that I can't sleep well on that single occasion. Just close your eyes for a while and if you don't feel like sleeping, do something else. If you just say to yourself after you arrive in London that whatever sleep you got was good enough because had you really needed a lot more sleep you would have slept more, then you will do a lot better than if you focus on how drowsy you feel and link that to not getting much sleep on the plane. See also this study.

The following does not constitute medical advice.

I find diphenhydramine hydrochloride is useful in my travels for when I absolutely must get some sleep before arrival. It is an antihistamine usually sold under a trade name such as "Nytol" (UK) or "Benadril" (South Africa, USA). In my experience it plays well with food and the moderate quantity of alcohol that attends any dinner (although excessive alcohol consumption should be avoided). It also has a relatively short biological half life so I do wake feeling quite refreshed. However on the eastbound transatlantic you will struggle to get in more than six hours so you may be drowsy for a few hours after landing.

The only side effect is that, as an antihistamine it does leave your mouth very dry, and this is redoubled by the effect of the low humidity air onboard airliners. Have plenty of water ready for when you wake up. There is also a mild dependence aspect from repeated use but this should not be a problem unless you are going to be shuttling between the USA and Europe several times a week.

Obviously, read the packaging carefully and if necessary consult with your doctor.

Aside from this, my advice is eat in the lounge or the terminal well before boarding, so once on board you can lay out and fall asleep shortly after take off. You will see the weekly commuters on the NYC-LON route are asleep before take off and only wake on landing. Use the provided ear plugs and blindfolds too, even if they feel uncomfortable at first.